<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e4114-b1e9-75f7-86e6-46bb4cfb9eed,g:54e9ef08-4c9a-45ce-a6a1-941bb1b34590-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="simple" xlink:href="upst-20260630.xsd#CondensedConsolidatedBalanceSheetsParenthetical"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e4114-b5ac-7919-bb5f-61cf72d7fa55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_StatementTable_019e4114-b5ac-7919-bb5f-61cf72d7fa55" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ac-7f58-a3bc-b1300283656a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e4114-b5ac-7919-bb5f-61cf72d7fa55" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ac-7f58-a3bc-b1300283656a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ac-7f58-a3bc-b1300283656a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ac-7f58-a3bc-b1300283656a" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ac-7f58-a3bc-b1300283656a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ac-7122-88e2-ba5268d638bc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ac-7f58-a3bc-b1300283656a" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ac-7122-88e2-ba5268d638bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b5ac-737d-a6ac-1a52eb370245" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ac-7122-88e2-ba5268d638bc" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b5ac-737d-a6ac-1a52eb370245" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsFairValueDisclosure_019e4114-b5ac-702f-b26c-c68cdc928066" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_OtherAssetsFairValueDisclosure_019e4114-b5ac-702f-b26c-c68cdc928066" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AccruedExpensesAndOtherLiabilitiesFairValueDisclosure_019e4114-b5ac-7971-9812-6feb392c19ff" xlink:href="upst-20260630.xsd#upst_AccruedExpensesAndOtherLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_upst_AccruedExpensesAndOtherLiabilitiesFairValueDisclosure_019e4114-b5ac-7971-9812-6feb392c19ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e4114-b5ac-7059-b01c-09218bd1c18b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e4114-b5ac-7059-b01c-09218bd1c18b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019e4114-b5ac-713c-8a61-41d67880bcdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019e4114-b5ac-713c-8a61-41d67880bcdd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019e4114-b5ac-73b1-b86a-81c329e682cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_CommonStockSharesIssued_019e4114-b5ac-73b1-b86a-81c329e682cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e4114-b5ac-7bc1-8297-0e0429cbf122" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e4114-b5ac-7bc1-8297-0e0429cbf122" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansHeldInConsolidatedSecuritizationFairValueDisclosure_019e4114-b5ac-7c45-911c-4c4f4812c287" xlink:href="upst-20260630.xsd#upst_LoansHeldInConsolidatedSecuritizationFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_upst_LoansHeldInConsolidatedSecuritizationFairValueDisclosure_019e4114-b5ac-7c45-911c-4c4f4812c287" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityVIEOrPotentialVIEInformationUnavailabilityDisclosuresAbstract_019e4114-b5ac-7cb5-b872-63b95478a135" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityVIEOrPotentialVIEInformationUnavailabilityDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-730e-910f-ca286f8c61e8" xlink:to="loc_us-gaap_VariableInterestEntityVIEOrPotentialVIEInformationUnavailabilityDisclosuresAbstract_019e4114-b5ac-7cb5-b872-63b95478a135" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityVIEOrPotentialVIEInformationUnavailabilityDisclosuresAbstract_019e4114-b5ac-7cb5-b872-63b95478a135" xlink:to="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e4114-b5ac-73bf-9ff5-c6956d5fb3be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e4114-b5ac-73bf-9ff5-c6956d5fb3be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCash_019e4114-b5ac-76e8-b84c-e4077c99021d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:to="loc_us-gaap_RestrictedCash_019e4114-b5ac-76e8-b84c-e4077c99021d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableNet_019e4114-b5ac-74fb-bc6a-537e0aa84f06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:to="loc_us-gaap_NotesReceivableNet_019e4114-b5ac-74fb-bc6a-537e0aa84f06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssets_019e4114-b5ac-798c-897a-d3b20986ea22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:to="loc_us-gaap_OtherAssets_019e4114-b5ac-798c-897a-d3b20986ea22" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019e4114-b5ac-7057-8e73-553b037d95bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e4114-b5ac-7f9a-b77b-9ddc406d5a77" xlink:to="loc_us-gaap_Assets_019e4114-b5ac-7057-8e73-553b037d95bd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityVIEOrPotentialVIEInformationUnavailabilityDisclosuresAbstract_019e4114-b5ac-7cb5-b872-63b95478a135" xlink:to="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PayableToInvestors_019e4114-b5ac-7c3b-bdfc-5e6dbdba7d21" xlink:href="upst-20260630.xsd#upst_PayableToInvestors"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:to="loc_upst_PayableToInvestors_019e4114-b5ac-7c3b-bdfc-5e6dbdba7d21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019e4114-b5ac-7ced-ad48-b6834c344320" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:to="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019e4114-b5ac-7ced-ad48-b6834c344320" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PayableToSecuritizationNoteHoldersAtFairValue_019e4114-b5ac-789d-8b06-e567dc0fdc35" xlink:href="upst-20260630.xsd#upst_PayableToSecuritizationNoteHoldersAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:to="loc_upst_PayableToSecuritizationNoteHoldersAtFairValue_019e4114-b5ac-789d-8b06-e567dc0fdc35" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_019e4114-b5ac-7ff0-814e-0742cf1093cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedLiabilitiesAndOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:to="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_019e4114-b5ac-7ff0-814e-0742cf1093cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019e4114-b5ac-77aa-b7eb-549e615f2b44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019e4114-b5ac-7dd2-a5c9-e56c73fdb740" xlink:to="loc_us-gaap_Liabilities_019e4114-b5ac-77aa-b7eb-549e615f2b44" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e4114-b5ac-78c0-8bff-00f035e8466d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityVIEOrPotentialVIEInformationUnavailabilityDisclosuresAbstract_019e4114-b5ac-7cb5-b872-63b95478a135" xlink:to="loc_us-gaap_StockholdersEquity_019e4114-b5ac-78c0-8bff-00f035e8466d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="upst-20260630.xsd#CondensedConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e4114-b5ac-78f7-8ed4-c441d61bcc01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e4114-b5ac-7043-a011-549ad3152e3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-78f7-8ed4-c441d61bcc01" xlink:to="loc_us-gaap_StatementTable_019e4114-b5ac-7043-a011-549ad3152e3c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b5ac-7340-bc16-88806084340d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e4114-b5ac-7043-a011-549ad3152e3c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b5ac-7340-bc16-88806084340d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-7340-bc16-88806084340d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b5ac-7340-bc16-88806084340d" xlink:to="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-7340-bc16-88806084340d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-75a5-bcdf-8d6e351f9b97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b5ac-7340-bc16-88806084340d" xlink:to="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-75a5-bcdf-8d6e351f9b97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e4114-b5ac-7389-9b20-0582246d8313" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-75a5-bcdf-8d6e351f9b97" xlink:to="loc_us-gaap_CommonStockMember_019e4114-b5ac-7389-9b20-0582246d8313" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e4114-b5ac-7b21-af49-15c0f82f4b21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-75a5-bcdf-8d6e351f9b97" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e4114-b5ac-7b21-af49-15c0f82f4b21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e4114-b5ac-7255-b8a6-9a64ea27edff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4114-b5ac-75a5-bcdf-8d6e351f9b97" xlink:to="loc_us-gaap_RetainedEarningsMember_019e4114-b5ac-7255-b8a6-9a64ea27edff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4114-b5ac-78f7-8ed4-c441d61bcc01" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e4114-b5ac-79f0-90bb-315a4af88abc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e4114-b5ac-79f0-90bb-315a4af88abc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e4114-b5ac-7881-9a74-e1508a820594" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockholdersEquity_019e4114-b5ac-7881-9a74-e1508a820594" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e4114-b5ac-72fc-a3f6-85faa6e3a5f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e4114-b5ac-72fc-a3f6-85faa6e3a5f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e4114-b5ac-7fa2-b962-f795b70eadd2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e4114-b5ac-7fa2-b962-f795b70eadd2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_019e4114-b5ac-717f-b49b-ab13f13868ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_019e4114-b5ac-717f-b49b-ab13f13868ce" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e4114-b5ac-7e98-ac83-7245e605bee6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e4114-b5ac-7e98-ac83-7245e605bee6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e4114-b5ac-72e6-9c59-084459289b3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e4114-b5ac-72e6-9c59-084459289b3d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e4114-b5ac-7ed4-a6ac-5b440a6d33da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e4114-b5ac-7ed4-a6ac-5b440a6d33da" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e4114-b5ac-7120-9e13-60e252f2bd5e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e4114-b5ac-7120-9e13-60e252f2bd5e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019e4114-b5ac-746e-8cb6-235bfdfb9076" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019e4114-b5ac-746e-8cb6-235bfdfb9076" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_019e4114-b5ac-7180-bd6a-4c812588650f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_019e4114-b5ac-7180-bd6a-4c812588650f" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_StockRepurchasedDuringPeriodValueAdjustment_019fb490-d837-7a21-ae5a-40b25d0b90a5" xlink:href="upst-20260630.xsd#upst_StockRepurchasedDuringPeriodValueAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_upst_StockRepurchasedDuringPeriodValueAdjustment_019fb490-d837-7a21-ae5a-40b25d0b90a5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_019e4114-b5ac-7f3b-8882-039f54fb3c9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_019e4114-b5ac-7f3b-8882-039f54fb3c9c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e4114-b5ac-772e-b115-f90b29d66aef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4114-b5ac-79b3-8544-d50b46ae0e85" xlink:to="loc_us-gaap_NetIncomeLoss_019e4114-b5ac-772e-b115-f90b29d66aef" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e4114-b5ac-760e-9f36-50f6564c6320" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e4114-b5ac-7637-b1fa-ea70712f93da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/RevenueScheduleofDisaggregationofRevenuebyTypeofServiceDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#RevenueScheduleofDisaggregationofRevenuebyTypeofServiceDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/RevenueScheduleofDisaggregationofRevenuebyTypeofServiceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7084-b69d-f786a2c34bc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-79c4-9de7-c30e7d7b07e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7084-b69d-f786a2c34bc7" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-79c4-9de7-c30e7d7b07e9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7992-bcf6-b60a406fbb15" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-79c4-9de7-c30e7d7b07e9" xlink:to="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7992-bcf6-b60a406fbb15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7992-bcf6-b60a406fbb15_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7992-bcf6-b60a406fbb15" xlink:to="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7992-bcf6-b60a406fbb15_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7936-8616-c09706056c51" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7992-bcf6-b60a406fbb15" xlink:to="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7936-8616-c09706056c51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PlatformAndReferralFeesNetMember_019e4114-b5ad-718c-b3a3-4b5f0d956ff3" xlink:href="upst-20260630.xsd#upst_PlatformAndReferralFeesNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7936-8616-c09706056c51" xlink:to="loc_upst_PlatformAndReferralFeesNetMember_019e4114-b5ad-718c-b3a3-4b5f0d956ff3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ServicingFeesNetMember_019e4114-b5ad-7e2e-97b8-3bc45c22294f" xlink:href="upst-20260630.xsd#upst_ServicingFeesNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7936-8616-c09706056c51" xlink:to="loc_upst_ServicingFeesNetMember_019e4114-b5ad-7e2e-97b8-3bc45c22294f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoanSalesFeesMember_019e9147-20b5-7d38-b7f8-c825ceca9a52" xlink:href="upst-20260630.xsd#upst_LoanSalesFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7936-8616-c09706056c51" xlink:to="loc_upst_LoanSalesFeesMember_019e9147-20b5-7d38-b7f8-c825ceca9a52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RevenueFromFeesNet_019e4114-b5ad-73ac-be33-9864a4da48d1" xlink:href="upst-20260630.xsd#upst_RevenueFromFeesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7084-b69d-f786a2c34bc7" xlink:to="loc_upst_RevenueFromFeesNet_019e4114-b5ad-73ac-be33-9864a4da48d1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/RevenueNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-73fd-af79-8feab63ce87d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-73fd-af79-8feab63ce87d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e4114-b5ad-77a5-9e30-8d203fb6b82b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-73fd-af79-8feab63ce87d" xlink:to="loc_srt_ProductOrServiceAxis_019e4114-b5ad-77a5-9e30-8d203fb6b82b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-77a5-9e30-8d203fb6b82b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e4114-b5ad-77a5-9e30-8d203fb6b82b" xlink:to="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-77a5-9e30-8d203fb6b82b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-727e-8f97-fb92c8b52497" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e4114-b5ad-77a5-9e30-8d203fb6b82b" xlink:to="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-727e-8f97-fb92c8b52497" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PlatformAndReferralFeesNetMember_019e4114-b5ad-7a98-8027-f5b6a67870e6" xlink:href="upst-20260630.xsd#upst_PlatformAndReferralFeesNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-727e-8f97-fb92c8b52497" xlink:to="loc_upst_PlatformAndReferralFeesNetMember_019e4114-b5ad-7a98-8027-f5b6a67870e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_OriginationFeesMember_019f84f4-88c8-76a5-9788-9c7bba7fb004" xlink:href="upst-20260630.xsd#upst_OriginationFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-727e-8f97-fb92c8b52497" xlink:to="loc_upst_OriginationFeesMember_019f84f4-88c8-76a5-9788-9c7bba7fb004" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e4114-b5ad-7b75-a6b3-e3dd9d4e400a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-73fd-af79-8feab63ce87d" xlink:to="loc_srt_RangeAxis_019e4114-b5ad-7b75-a6b3-e3dd9d4e400a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b5ad-7b75-a6b3-e3dd9d4e400a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e4114-b5ad-7b75-a6b3-e3dd9d4e400a" xlink:to="loc_srt_RangeMember_019e4114-b5ad-7b75-a6b3-e3dd9d4e400a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b5ad-70df-91be-170795c48537" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e4114-b5ad-7b75-a6b3-e3dd9d4e400a" xlink:to="loc_srt_RangeMember_019e4114-b5ad-70df-91be-170795c48537" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e4114-b5ad-7c3f-9509-391c752e7bbe" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b5ad-70df-91be-170795c48537" xlink:to="loc_srt_MinimumMember_019e4114-b5ad-7c3f-9509-391c752e7bbe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e4114-b5ad-7d00-a971-1c806bcf5c60" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b5ad-70df-91be-170795c48537" xlink:to="loc_srt_MaximumMember_019e4114-b5ad-7d00-a971-1c806bcf5c60" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b5ad-7f71-874b-44a87e934137" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-73fd-af79-8feab63ce87d" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b5ad-7f71-874b-44a87e934137" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b5ad-7f71-874b-44a87e934137_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b5ad-7f71-874b-44a87e934137" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b5ad-7f71-874b-44a87e934137_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b5ad-7e9f-93bc-ff14a351c27a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b5ad-7f71-874b-44a87e934137" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b5ad-7e9f-93bc-ff14a351c27a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HomeEquityMember_019e4114-b5ad-7c5b-85e7-b73fe0e63ecb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HomeEquityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b5ad-7e9f-93bc-ff14a351c27a" xlink:to="loc_us-gaap_HomeEquityMember_019e4114-b5ad-7c5b-85e7-b73fe0e63ecb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b5ad-72ea-8551-521e0c3b5bec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-73fd-af79-8feab63ce87d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b5ad-72ea-8551-521e0c3b5bec" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b5ad-72ea-8551-521e0c3b5bec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b5ad-72ea-8551-521e0c3b5bec" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b5ad-72ea-8551-521e0c3b5bec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b5ad-7162-9c1f-6e186b3a05cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b5ad-72ea-8551-521e0c3b5bec" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b5ad-7162-9c1f-6e186b3a05cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e4114-b5ad-7f91-9e2b-faeb7394f9f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b5ad-7162-9c1f-6e186b3a05cd" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e4114-b5ad-7f91-9e2b-faeb7394f9f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoanPremiumFeesAndLoanTrailingFees_019ef8fe-035a-7dd5-a9a7-75c6b3f09c86" xlink:href="upst-20260630.xsd#upst_LoanPremiumFeesAndLoanTrailingFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_upst_LoanPremiumFeesAndLoanTrailingFees_019ef8fe-035a-7dd5-a9a7-75c6b3f09c86" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TrailingFeeLiabilitiesFairValueDisclosure_019e4114-b5ad-73ab-bb53-ad6bfb42b238" xlink:href="upst-20260630.xsd#upst_TrailingFeeLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_upst_TrailingFeeLiabilitiesFairValueDisclosure_019e4114-b5ad-73ab-bb53-ad6bfb42b238" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SubscriptionAgreementsTerm_019e4114-b5ad-7f8e-a115-8037d57f64bf" xlink:href="upst-20260630.xsd#upst_SubscriptionAgreementsTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_upst_SubscriptionAgreementsTerm_019e4114-b5ad-7f8e-a115-8037d57f64bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f84f3-bfb5-70b9-9e32-830715dfd407" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f84f3-bfb5-70b9-9e32-830715dfd407" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedFeesAndOtherRevenueReceivable_019e4114-b5ad-73a5-b09d-284cca5cb396" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedFeesAndOtherRevenueReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_AccruedFeesAndOtherRevenueReceivable_019e4114-b5ad-73a5-b09d-284cca5cb396" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_019e4114-b5ad-736d-b627-b301ea3eb017" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_019e4114-b5ad-736d-b627-b301ea3eb017" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableThresholdPeriodPastDue_019e4114-b5ad-711d-aa3d-ce3c34f0e389" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableThresholdPeriodPastDue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_FinancingReceivableThresholdPeriodPastDue_019e4114-b5ad-711d-aa3d-ce3c34f0e389" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss_019e4114-b5ad-75b1-b7f5-216a275e9ca4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss_019e4114-b5ad-75b1-b7f5-216a275e9ca4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendIncomeOperating_019e4114-b5ad-764f-ae28-ec5256ead2fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendIncomeOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_DividendIncomeOperating_019e4114-b5ad-764f-ae28-ec5256ead2fc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019e4114-b5ad-74c8-9987-a601588dfba7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_InterestExpenseDebt_019e4114-b5ad-74c8-9987-a601588dfba7" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromRecoveriesOfLoanPreviouslyChargedOff_019ef90e-66a4-7dfd-9ef8-63d470bea2b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromRecoveriesOfLoanPreviouslyChargedOff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-74ac-a5fd-363a5c26d6c3" xlink:to="loc_us-gaap_ProceedsFromRecoveriesOfLoanPreviouslyChargedOff_019ef90e-66a4-7dfd-9ef8-63d470bea2b9" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/RevenueScheduleofCustomersAccountingforGreaterThan10ofRevenueandAccountsReceivableDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#RevenueScheduleofCustomersAccountingforGreaterThan10ofRevenueandAccountsReceivableDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/RevenueScheduleofCustomersAccountingforGreaterThan10ofRevenueandAccountsReceivableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7e84-9b72-8d7b32a750ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7b58-a389-c3572f3ffea8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7e84-9b72-8d7b32a750ca" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7b58-a389-c3572f3ffea8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019e4114-b5ad-7c9b-89ca-47bf1bcebcad" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7b58-a389-c3572f3ffea8" xlink:to="loc_srt_MajorCustomersAxis_019e4114-b5ad-7c9b-89ca-47bf1bcebcad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7c9b-89ca-47bf1bcebcad_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019e4114-b5ad-7c9b-89ca-47bf1bcebcad" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7c9b-89ca-47bf1bcebcad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019e4114-b5ad-7c9b-89ca-47bf1bcebcad" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CustomerAMember_019e4114-b5ad-7d60-8435-aacbedbd3972" xlink:href="upst-20260630.xsd#upst_CustomerAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:to="loc_upst_CustomerAMember_019e4114-b5ad-7d60-8435-aacbedbd3972" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CustomerBMember_019e4114-b5ad-7fc6-92ce-c69116abfce2" xlink:href="upst-20260630.xsd#upst_CustomerBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:to="loc_upst_CustomerBMember_019e4114-b5ad-7fc6-92ce-c69116abfce2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CustomerCMember_019e4114-b5ad-7a39-9759-c1a14d6a5edf" xlink:href="upst-20260630.xsd#upst_CustomerCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:to="loc_upst_CustomerCMember_019e4114-b5ad-7a39-9759-c1a14d6a5edf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CustomerDMember_019e4114-b5ad-7aa4-948c-3a2bdd8ea1cc" xlink:href="upst-20260630.xsd#upst_CustomerDMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:to="loc_upst_CustomerDMember_019e4114-b5ad-7aa4-948c-3a2bdd8ea1cc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CustomerEMember_019ef904-e471-7fc5-a20d-6fed2531ea37" xlink:href="upst-20260630.xsd#upst_CustomerEMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e4114-b5ad-7ea4-b887-3cf9a132f660" xlink:to="loc_upst_CustomerEMember_019ef904-e471-7fc5-a20d-6fed2531ea37" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e4114-b5ad-73b0-a1d7-b1ca245c8033" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7b58-a389-c3572f3ffea8" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e4114-b5ad-73b0-a1d7-b1ca245c8033" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e4114-b5ad-73b0-a1d7-b1ca245c8033_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e4114-b5ad-73b0-a1d7-b1ca245c8033" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e4114-b5ad-73b0-a1d7-b1ca245c8033_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e4114-b5ad-74c6-8dda-1e0ad96b2dfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e4114-b5ad-73b0-a1d7-b1ca245c8033" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e4114-b5ad-74c6-8dda-1e0ad96b2dfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_019e4114-b5ad-7f4c-8d53-5b51d2ab3139" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e4114-b5ad-74c6-8dda-1e0ad96b2dfb" xlink:to="loc_us-gaap_SalesRevenueNetMember_019e4114-b5ad-7f4c-8d53-5b51d2ab3139" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019e4114-b5ad-7a9b-b405-3c45d6f4df8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e4114-b5ad-74c6-8dda-1e0ad96b2dfb" xlink:to="loc_us-gaap_AccountsReceivableMember_019e4114-b5ad-7a9b-b405-3c45d6f4df8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019e4114-b5ad-7959-873d-b5b95ae5ef6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7b58-a389-c3572f3ffea8" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019e4114-b5ad-7959-873d-b5b95ae5ef6d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e4114-b5ad-7959-873d-b5b95ae5ef6d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e4114-b5ad-7959-873d-b5b95ae5ef6d" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e4114-b5ad-7959-873d-b5b95ae5ef6d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e4114-b5ad-75be-9630-ffccde200111" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e4114-b5ad-7959-873d-b5b95ae5ef6d" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e4114-b5ad-75be-9630-ffccde200111" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019e4114-b5ad-773b-9a41-295f27a25cac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e4114-b5ad-75be-9630-ffccde200111" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019e4114-b5ad-773b-9a41-295f27a25cac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019e4114-b5ad-7300-8a05-14c0a0709112" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7e84-9b72-8d7b32a750ca" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019e4114-b5ad-7300-8a05-14c0a0709112" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/RevenueScheduleofCollectionAgencyandBorrowerFeesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#RevenueScheduleofCollectionAgencyandBorrowerFeesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/RevenueScheduleofCollectionAgencyandBorrowerFeesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7db5-99e5-c5bc79b83a9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-72f7-998c-e54b2a7a5902" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7db5-99e5-c5bc79b83a9b" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-72f7-998c-e54b2a7a5902" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7892-89c4-0b2a8068abc3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-72f7-998c-e54b2a7a5902" xlink:to="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7892-89c4-0b2a8068abc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7892-89c4-0b2a8068abc3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7892-89c4-0b2a8068abc3" xlink:to="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-7892-89c4-0b2a8068abc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e4114-b5ad-7892-89c4-0b2a8068abc3" xlink:to="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ServicingFeesMember_019e4114-b5ad-7182-b1aa-461b1ba1de01" xlink:href="upst-20260630.xsd#upst_ServicingFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:to="loc_upst_ServicingFeesMember_019e4114-b5ad-7182-b1aa-461b1ba1de01" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BorrowerFeesMember_019e4114-b5ad-7fae-b723-ce91c8d1b160" xlink:href="upst-20260630.xsd#upst_BorrowerFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:to="loc_upst_BorrowerFeesMember_019e4114-b5ad-7fae-b723-ce91c8d1b160" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CollectionAgencyFeesMember_019e4114-b5ad-7d48-83a1-92f6c6fd69d6" xlink:href="upst-20260630.xsd#upst_CollectionAgencyFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:to="loc_upst_CollectionAgencyFeesMember_019e4114-b5ad-7d48-83a1-92f6c6fd69d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_OtherFeesMember_019e4114-b5ad-7097-8ad4-9e416a14e382" xlink:href="upst-20260630.xsd#upst_OtherFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:to="loc_upst_OtherFeesMember_019e4114-b5ad-7097-8ad4-9e416a14e382" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ServicingFeesNetMember_019e4114-b5ad-74c3-a33e-44b3feae21bc" xlink:href="upst-20260630.xsd#upst_ServicingFeesNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4114-b5ad-785b-a323-be94286e6c7e" xlink:to="loc_upst_ServicingFeesNetMember_019e4114-b5ad-74c3-a33e-44b3feae21bc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RevenueFromFeesNet_019e4114-b5ad-7372-af4e-fb577408ea04" xlink:href="upst-20260630.xsd#upst_RevenueFromFeesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7db5-99e5-c5bc79b83a9b" xlink:to="loc_upst_RevenueFromFeesNet_019e4114-b5ad-7372-af4e-fb577408ea04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_GainLossOnServicingRightsAndFairValueAdjustments_019e4114-b5ad-7c4a-8de6-674ee4f890af" xlink:href="upst-20260630.xsd#upst_GainLossOnServicingRightsAndFairValueAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7db5-99e5-c5bc79b83a9b" xlink:to="loc_upst_GainLossOnServicingRightsAndFairValueAdjustments_019e4114-b5ad-7c4a-8de6-674ee4f890af" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/RevenueScheduleofComponentsofInterestIncomeandFairValueAdjustmentsNetDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#RevenueScheduleofComponentsofInterestIncomeandFairValueAdjustmentsNetDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/RevenueScheduleofComponentsofInterestIncomeandFairValueAdjustmentsNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7625-a1d2-fce93c5a584e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7625-a1d2-fce93c5a584e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7529-8ef7-0089f808c016" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e4114-b5ad-7625-a1d2-fce93c5a584e" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7529-8ef7-0089f808c016" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7529-8ef7-0089f808c016_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7529-8ef7-0089f808c016" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7529-8ef7-0089f808c016_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-73e9-a5e3-a92976d19509" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7529-8ef7-0089f808c016" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-73e9-a5e3-a92976d19509" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b5ad-7f2b-919a-eee0d5eb06c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-73e9-a5e3-a92976d19509" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b5ad-7f2b-919a-eee0d5eb06c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeOperating_019e4114-b5ad-72b4-98b1-74f793da4b38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_us-gaap_InterestIncomeOperating_019e4114-b5ad-72b4-98b1-74f793da4b38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseOperating_019e4114-b5ad-714d-9b9e-195c9bb54cfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_us-gaap_InterestExpenseOperating_019e4114-b5ad-714d-9b9e-195c9bb54cfc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_UnrealizedGainLossChargeOffsAndOtherFairValueAdjustmentsNet_019e4114-b5ad-7d03-acdb-244f1fb92ff7" xlink:href="upst-20260630.xsd#upst_UnrealizedGainLossChargeOffsAndOtherFairValueAdjustmentsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_upst_UnrealizedGainLossChargeOffsAndOtherFairValueAdjustmentsNet_019e4114-b5ad-7d03-acdb-244f1fb92ff7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_GainLossOnBeneficialInterests_019e4114-b5ad-75d4-a701-b47caa734ef2" xlink:href="upst-20260630.xsd#upst_GainLossOnBeneficialInterests"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_upst_GainLossOnBeneficialInterests_019e4114-b5ad-75d4-a701-b47caa734ef2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RealizedGainLossOnTransferOfLoansNet_019e4114-b5ad-7f11-8a32-0e6c03d1e251" xlink:href="upst-20260630.xsd#upst_RealizedGainLossOnTransferOfLoansNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_upst_RealizedGainLossOnTransferOfLoansNet_019e4114-b5ad-7f11-8a32-0e6c03d1e251" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueAndOtherAdjustmentsNet_019e4114-b5ad-7e80-911e-5301151f07d5" xlink:href="upst-20260630.xsd#upst_FairValueAndOtherAdjustmentsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_upst_FairValueAndOtherAdjustmentsNet_019e4114-b5ad-7e80-911e-5301151f07d5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_InterestIncomeAndFairValueAdjustmentNet_019e4114-b5ad-761e-b8fb-1498e4cd15af" xlink:href="upst-20260630.xsd#upst_InterestIncomeAndFairValueAdjustmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e4114-b5ad-7d9d-ae64-919e71db4510" xlink:to="loc_upst_InterestIncomeAndFairValueAdjustmentNet_019e4114-b5ad-761e-b8fb-1498e4cd15af" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/VariableInterestEntitiesScheduleofFinancialAssetsandLiabilitiesfromVariableInterestEntitiesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#VariableInterestEntitiesScheduleofFinancialAssetsandLiabilitiesfromVariableInterestEntitiesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/VariableInterestEntitiesScheduleofFinancialAssetsandLiabilitiesfromVariableInterestEntitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7fcd-b8e8-a00711c65409" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-7721-8052-8c1500a045a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7fcd-b8e8-a00711c65409" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-7721-8052-8c1500a045a4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-779d-bf8d-bbfe3ffdc150" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-7721-8052-8c1500a045a4" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-779d-bf8d-bbfe3ffdc150" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-779d-bf8d-bbfe3ffdc150_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-779d-bf8d-bbfe3ffdc150" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-779d-bf8d-bbfe3ffdc150_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7874-8f89-06a27fc9fa87" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-779d-bf8d-bbfe3ffdc150" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7874-8f89-06a27fc9fa87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b5ad-7d28-808d-ffe2f9a9866c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7874-8f89-06a27fc9fa87" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b5ad-7d28-808d-ffe2f9a9866c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_019e4114-b5ad-7889-bc76-af8b49120030" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7874-8f89-06a27fc9fa87" xlink:to="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_019e4114-b5ad-7889-bc76-af8b49120030" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019e4114-b5ad-71a6-8f88-c8943ebd6df1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-7721-8052-8c1500a045a4" xlink:to="loc_srt_CounterpartyNameAxis_019e4114-b5ad-71a6-8f88-c8943ebd6df1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-71a6-8f88-c8943ebd6df1_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019e4114-b5ad-71a6-8f88-c8943ebd6df1" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-71a6-8f88-c8943ebd6df1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-7b20-9b61-2bd1742cb873" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019e4114-b5ad-71a6-8f88-c8943ebd6df1" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-7b20-9b61-2bd1742cb873" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConsolidatedSecuritizationMember_019e4114-b5ad-7a53-aa96-774e479e30d7" xlink:href="upst-20260630.xsd#upst_ConsolidatedSecuritizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-7b20-9b61-2bd1742cb873" xlink:to="loc_upst_ConsolidatedSecuritizationMember_019e4114-b5ad-7a53-aa96-774e479e30d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConsolidatedWarehouseEntitiesMember_019e4114-b5ad-75bf-baac-2b9fa3676c14" xlink:href="upst-20260630.xsd#upst_ConsolidatedWarehouseEntitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-7b20-9b61-2bd1742cb873" xlink:to="loc_upst_ConsolidatedWarehouseEntitiesMember_019e4114-b5ad-75bf-baac-2b9fa3676c14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_OtherVariableInterestEntitiesMember_019e4114-b5ad-7005-8708-2566d7daead0" xlink:href="upst-20260630.xsd#upst_OtherVariableInterestEntitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-7b20-9b61-2bd1742cb873" xlink:to="loc_upst_OtherVariableInterestEntitiesMember_019e4114-b5ad-7005-8708-2566d7daead0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SecuritizationsMember_019e4114-b5ad-7931-bdec-351c9856d357" xlink:href="upst-20260630.xsd#upst_SecuritizationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e4114-b5ad-7b20-9b61-2bd1742cb873" xlink:to="loc_upst_SecuritizationsMember_019e4114-b5ad-7931-bdec-351c9856d357" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019e4114-b5ad-7f4c-a54e-ebe23cc33fd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7fcd-b8e8-a00711c65409" xlink:to="loc_us-gaap_Assets_019e4114-b5ad-7f4c-a54e-ebe23cc33fd8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019e4114-b5ad-7d0b-b5c3-ec8d1674afec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7fcd-b8e8-a00711c65409" xlink:to="loc_us-gaap_Liabilities_019e4114-b5ad-7d0b-b5c3-ec8d1674afec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e4114-b5ad-731e-bdfb-193745f26070" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7fcd-b8e8-a00711c65409" xlink:to="loc_us-gaap_StockholdersEquity_019e4114-b5ad-731e-bdfb-193745f26070" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount_019e4114-b5ad-7cbc-b862-8780e5e3f6e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7fcd-b8e8-a00711c65409" xlink:to="loc_us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount_019e4114-b5ad-7cbc-b862-8780e5e3f6e4" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/VariableInterestEntitiesNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#VariableInterestEntitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/VariableInterestEntitiesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7bc2-a596-58a1ec888eb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-747e-87aa-3b493b79dad1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7bc2-a596-58a1ec888eb9" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-747e-87aa-3b493b79dad1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7f8d-8ebd-4c02d032b5a0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-747e-87aa-3b493b79dad1" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7f8d-8ebd-4c02d032b5a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7f8d-8ebd-4c02d032b5a0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7f8d-8ebd-4c02d032b5a0" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-7f8d-8ebd-4c02d032b5a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-70d1-b24a-0eb7a61dc306" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b5ad-7f8d-8ebd-4c02d032b5a0" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-70d1-b24a-0eb7a61dc306" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_019e4114-b5ad-770f-a01d-bda2cc528fe0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019e4114-b5ad-70d1-b24a-0eb7a61dc306" xlink:to="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_019e4114-b5ad-770f-a01d-bda2cc528fe0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b5ad-78b4-aa1d-706831470fc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019e4114-b5ad-747e-87aa-3b493b79dad1" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b5ad-78b4-aa1d-706831470fc7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019e4114-b5ad-78b4-aa1d-706831470fc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b5ad-78b4-aa1d-706831470fc7" xlink:to="loc_us-gaap_ReceivableTypeDomain_019e4114-b5ad-78b4-aa1d-706831470fc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019e4114-b5ad-7395-8e6a-f4d317f6bc3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b5ad-78b4-aa1d-706831470fc7" xlink:to="loc_us-gaap_ReceivableTypeDomain_019e4114-b5ad-7395-8e6a-f4d317f6bc3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_NotesReceivableAndResidualCertificatesMember_019e4114-b5ad-7af3-9b58-953928589c3f" xlink:href="upst-20260630.xsd#upst_NotesReceivableAndResidualCertificatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019e4114-b5ad-7395-8e6a-f4d317f6bc3e" xlink:to="loc_upst_NotesReceivableAndResidualCertificatesMember_019e4114-b5ad-7af3-9b58-953928589c3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableNet_019e4114-b5ad-7903-9c85-2fb1e9e63ca0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019e4114-b5ad-7bc2-a596-58a1ec888eb9" xlink:to="loc_us-gaap_NotesReceivableNet_019e4114-b5ad-7903-9c85-2fb1e9e63ca0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BeneficialInterestsScheduleofBeneficialInterestAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BeneficialInterestsScheduleofBeneficialInterestAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BeneficialInterestsScheduleofBeneficialInterestAssetsandLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:href="upst-20260630.xsd#upst_ScheduleOfBeneficialInterestLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-7e4c-848f-8c0be04f7a23" xlink:href="upst-20260630.xsd#upst_ScheduleOfBeneficialInterestTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:to="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-7e4c-848f-8c0be04f7a23" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-7eec-9828-77c7be4c09a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-7e4c-848f-8c0be04f7a23" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-7eec-9828-77c7be4c09a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7eec-9828-77c7be4c09a9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-7eec-9828-77c7be4c09a9" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7eec-9828-77c7be4c09a9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7f40-b53e-6283de441b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-7eec-9828-77c7be4c09a9" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7f40-b53e-6283de441b2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HybridInstrumentMember_019e4114-b5ad-7720-86c4-7f28a989872d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HybridInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7f40-b53e-6283de441b2e" xlink:to="loc_us-gaap_HybridInstrumentMember_019e4114-b5ad-7720-86c4-7f28a989872d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeMember_019e4114-b5ad-725a-9211-7884f1339509" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7f40-b53e-6283de441b2e" xlink:to="loc_us-gaap_DerivativeMember_019e4114-b5ad-725a-9211-7884f1339509" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestsNetMember_019e4114-b5ad-7f1d-91c5-bbd30307440f" xlink:href="upst-20260630.xsd#upst_BeneficialInterestsNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-7f40-b53e-6283de441b2e" xlink:to="loc_upst_BeneficialInterestsNetMember_019e4114-b5ad-7f1d-91c5-bbd30307440f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b5ad-752a-a6fb-52db5bda64c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-7e4c-848f-8c0be04f7a23" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b5ad-752a-a6fb-52db5bda64c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b5ad-752a-a6fb-52db5bda64c3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b5ad-752a-a6fb-52db5bda64c3" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b5ad-752a-a6fb-52db5bda64c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b5ad-749e-87e0-1e27a96d75d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b5ad-752a-a6fb-52db5bda64c3" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b5ad-749e-87e0-1e27a96d75d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeMember_019e4114-b5ad-73f7-b931-2ac12e3a1296" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b5ad-749e-87e0-1e27a96d75d2" xlink:to="loc_us-gaap_DerivativeMember_019e4114-b5ad-73f7-b931-2ac12e3a1296" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestAssetUnpaidPrincipalBalance_019e4114-b5ad-7edd-8486-cf28341d693a" xlink:href="upst-20260630.xsd#upst_BeneficialInterestAssetUnpaidPrincipalBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:to="loc_upst_BeneficialInterestAssetUnpaidPrincipalBalance_019e4114-b5ad-7edd-8486-cf28341d693a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilityUnpaidPrincipalBalance_019e4114-b5ad-7b03-af46-e2142602c33a" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilityUnpaidPrincipalBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:to="loc_upst_BeneficialInterestLiabilityUnpaidPrincipalBalance_019e4114-b5ad-7b03-af46-e2142602c33a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestAsset_019e4114-b5ad-7e8c-b361-d9dc1523b521" xlink:href="upst-20260630.xsd#upst_BeneficialInterestAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:to="loc_upst_BeneficialInterestAsset_019e4114-b5ad-7e8c-b361-d9dc1523b521" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilities_019e4114-b5ad-74e1-8928-855c5d0803aa" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:to="loc_upst_BeneficialInterestLiabilities_019e4114-b5ad-74e1-8928-855c5d0803aa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019e4114-b5ad-7517-8202-809d38e4f83b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-7323-a130-5ace2b9e83e7" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019e4114-b5ad-7517-8202-809d38e4f83b" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BeneficialInterestsScheduleofMaximumExposuretoLossesbyAssetTypeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BeneficialInterestsScheduleofMaximumExposuretoLossesbyAssetTypeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BeneficialInterestsScheduleofMaximumExposuretoLossesbyAssetTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-70f9-a6af-9bb28a37c15d" xlink:href="upst-20260630.xsd#upst_ScheduleOfBeneficialInterestLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-736e-8359-16771ff2f56d" xlink:href="upst-20260630.xsd#upst_ScheduleOfBeneficialInterestTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-70f9-a6af-9bb28a37c15d" xlink:to="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-736e-8359-16771ff2f56d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-709d-a2ef-0c866c24ccaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBeneficialInterestTable_019e4114-b5ad-736e-8359-16771ff2f56d" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-709d-a2ef-0c866c24ccaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-709d-a2ef-0c866c24ccaa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-709d-a2ef-0c866c24ccaa" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-709d-a2ef-0c866c24ccaa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b5ad-709d-a2ef-0c866c24ccaa" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsMember_019e4114-b5ad-7aad-ae7c-dce9838d498f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:to="loc_us-gaap_CashAndCashEquivalentsMember_019e4114-b5ad-7aad-ae7c-dce9838d498f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RestrictedCashMember_019e4114-b5ad-7270-b803-1a419f05849d" xlink:href="upst-20260630.xsd#upst_RestrictedCashMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:to="loc_upst_RestrictedCashMember_019e4114-b5ad-7270-b803-1a419f05849d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestsNetMember_019e4114-b5ad-77ca-8131-ae183e243062" xlink:href="upst-20260630.xsd#upst_BeneficialInterestsNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:to="loc_upst_BeneficialInterestsNetMember_019e4114-b5ad-77ca-8131-ae183e243062" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditReceivableMember_019e4114-b5ad-71e4-8f1f-0f6dca9bb741" xlink:href="upst-20260630.xsd#upst_LineOfCreditReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:to="loc_upst_LineOfCreditReceivableMember_019e4114-b5ad-71e4-8f1f-0f6dca9bb741" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansMember_019e4114-b5ad-7443-b03d-110c2d0a7a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:to="loc_us-gaap_LoansMember_019e4114-b5ad-7443-b03d-110c2d0a7a6b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditReceivableAndBeneficialInterestAssetMember_019e4114-b5ad-760d-9053-cf2c6add84e8" xlink:href="upst-20260630.xsd#upst_LineOfCreditReceivableAndBeneficialInterestAssetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b5ad-76fa-ab1e-52fdb3d5ede7" xlink:to="loc_upst_LineOfCreditReceivableAndBeneficialInterestAssetMember_019e4114-b5ad-760d-9053-cf2c6add84e8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestsMaximumLossExposure_019e4114-b5ad-76c7-8706-b2ab53286e22" xlink:href="upst-20260630.xsd#upst_BeneficialInterestsMaximumLossExposure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-70f9-a6af-9bb28a37c15d" xlink:to="loc_upst_BeneficialInterestsMaximumLossExposure_019e4114-b5ad-76c7-8706-b2ab53286e22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AssetsHeldByInstitutionalInvestor_019e4114-b5ad-7f02-bfd3-046b99a721ec" xlink:href="upst-20260630.xsd#upst_AssetsHeldByInstitutionalInvestor"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBeneficialInterestLineItems_019e4114-b5ad-70f9-a6af-9bb28a37c15d" xlink:to="loc_upst_AssetsHeldByInstitutionalInvestor_019e4114-b5ad-7f02-bfd3-046b99a721ec" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofAssetsandLiabilitiesMeasuredatFairValueDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofAssetsandLiabilitiesMeasuredatFairValueDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofAssetsandLiabilitiesMeasuredatFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-71fe-b9fb-4118bb90b25c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67e-788b-867a-0936fa03f4a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-71fe-b9fb-4118bb90b25c" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67e-788b-867a-0936fa03f4a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67e-7e05-adff-86d27e3b2c90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67e-788b-867a-0936fa03f4a5" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67e-7e05-adff-86d27e3b2c90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67e-7e05-adff-86d27e3b2c90_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67e-7e05-adff-86d27e3b2c90" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67e-7e05-adff-86d27e3b2c90_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-781b-be5c-3f3999dc2b9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67e-7e05-adff-86d27e3b2c90" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-781b-be5c-3f3999dc2b9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-769c-b8ce-258dbecdb69d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-781b-be5c-3f3999dc2b9f" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-769c-b8ce-258dbecdb69d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e4114-b67f-7df2-988d-3ed3a79ce327" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-781b-be5c-3f3999dc2b9f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e4114-b67f-7df2-988d-3ed3a79ce327" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-71fe-b9fb-4118bb90b25c" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableFairValueDisclosure_019e4114-b67f-7ee7-acde-cd7ee99b5808" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_us-gaap_LoansReceivableFairValueDisclosure_019e4114-b67f-7ee7-acde-cd7ee99b5808" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestAssetFairValue_019e4114-b67f-7fe9-9ee9-473b5d75d14b" xlink:href="upst-20260630.xsd#upst_BeneficialInterestAssetFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_upst_BeneficialInterestAssetFairValue_019e4114-b67f-7fe9-9ee9-473b5d75d14b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditReceivableFairValueDisclosure_019e4114-b67f-760f-823f-fc020994fe41" xlink:href="upst-20260630.xsd#upst_LineOfCreditReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_upst_LineOfCreditReceivableFairValueDisclosure_019e4114-b67f-760f-823f-fc020994fe41" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-77cc-bb98-9a13d93a57fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-77cc-bb98-9a13d93a57fd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableFairValueDisclosure_019e4114-b67f-79e5-9da2-a629aee3a0e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_us-gaap_NotesReceivableFairValueDisclosure_019e4114-b67f-79e5-9da2-a629aee3a0e3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativeAssetsAtFairValue_019e4114-b67f-764a-821c-048966e6438c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateDerivativeAssetsAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_us-gaap_InterestRateDerivativeAssetsAtFairValue_019e4114-b67f-764a-821c-048966e6438c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019e4114-b67f-77da-8269-c68580f56989" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4114-b67f-7822-bafc-9092e31431c4" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019e4114-b67f-77da-8269-c68580f56989" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-71fe-b9fb-4118bb90b25c" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PayableToSecuritizationNoteHoldersAtFairValue_019e4114-b67f-7711-a1ae-ef4205f2ace6" xlink:href="upst-20260630.xsd#upst_PayableToSecuritizationNoteHoldersAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:to="loc_upst_PayableToSecuritizationNoteHoldersAtFairValue_019e4114-b67f-7711-a1ae-ef4205f2ace6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilities_019e4114-b67f-7b80-b2a1-9c8450a00c6f" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:to="loc_upst_BeneficialInterestLiabilities_019e4114-b67f-7b80-b2a1-9c8450a00c6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TrailingFeeLiabilitiesFairValueDisclosure_019e4114-b67f-7415-a955-1ed707f4b73c" xlink:href="upst-20260630.xsd#upst_TrailingFeeLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:to="loc_upst_TrailingFeeLiabilitiesFairValueDisclosure_019e4114-b67f-7415-a955-1ed707f4b73c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-74fe-aeae-70d43ebfd587" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilityAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:to="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-74fe-aeae-70d43ebfd587" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_019e4114-b67f-71a3-bb4f-ca78c20ff340" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019e4114-b67f-7282-8243-c6ec7f1313fd" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_019e4114-b67f-71a3-bb4f-ca78c20ff340" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofFairValueofClassesofLoansHeldbytheCompanyDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofFairValueofClassesofLoansHeldbytheCompanyDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofFairValueofClassesofLoansHeldbytheCompanyDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7ce1-985b-5e615570b3f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7090-bf6a-91bf13e76aa6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7ce1-985b-5e615570b3f7" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7090-bf6a-91bf13e76aa6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7265-86f3-b354ce587655" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7090-bf6a-91bf13e76aa6" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7265-86f3-b354ce587655" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7265-86f3-b354ce587655_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7265-86f3-b354ce587655" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7265-86f3-b354ce587655_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7990-80f6-563c3791763f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7265-86f3-b354ce587655" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7990-80f6-563c3791763f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-704d-aa42-cf34b8949372" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7990-80f6-563c3791763f" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-704d-aa42-cf34b8949372" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansHeldForSaleFairValueDisclosure_019e4114-b67f-7f61-bb3e-b01614465fe1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansHeldForSaleFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7ce1-985b-5e615570b3f7" xlink:to="loc_us-gaap_LoansHeldForSaleFairValueDisclosure_019e4114-b67f-7f61-bb3e-b01614465fe1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansHeldForInvestmentFairValueDisclosure_019e4114-b67f-711c-8c6f-ffb6256698a4" xlink:href="upst-20260630.xsd#upst_LoansHeldForInvestmentFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7ce1-985b-5e615570b3f7" xlink:to="loc_upst_LoansHeldForInvestmentFairValueDisclosure_019e4114-b67f-711c-8c6f-ffb6256698a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansHeldInConsolidatedSecuritizationFairValueDisclosure_019e4114-b67f-7d93-8432-6e616a9c25d7" xlink:href="upst-20260630.xsd#upst_LoansHeldInConsolidatedSecuritizationFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7ce1-985b-5e615570b3f7" xlink:to="loc_upst_LoansHeldInConsolidatedSecuritizationFairValueDisclosure_019e4114-b67f-7d93-8432-6e616a9c25d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableFairValueDisclosure_019e4114-b67f-7f47-8daf-5c26f4ae6950" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7ce1-985b-5e615570b3f7" xlink:to="loc_us-gaap_LoansReceivableFairValueDisclosure_019e4114-b67f-7f47-8daf-5c26f4ae6950" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofSignificantUnobservableInputsDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofSignificantUnobservableInputsDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofSignificantUnobservableInputsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7bb7-830b-04550edbf89e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7bb7-830b-04550edbf89e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-756b-9ac5-522c4db054d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7bb7-830b-04550edbf89e" xlink:to="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-756b-9ac5-522c4db054d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-756b-9ac5-522c4db054d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-756b-9ac5-522c4db054d1" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-756b-9ac5-522c4db054d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-7639-b417-79acc55d7870" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-756b-9ac5-522c4db054d1" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-7639-b417-79acc55d7870" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDiscountedCashFlowMember_019e4114-b67f-78bd-8d5d-c6fc132d6e4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDiscountedCashFlowMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-7639-b417-79acc55d7870" xlink:to="loc_us-gaap_ValuationTechniqueDiscountedCashFlowMember_019e4114-b67f-78bd-8d5d-c6fc132d6e4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7091-b857-341edf28278a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7bb7-830b-04550edbf89e" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7091-b857-341edf28278a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7091-b857-341edf28278a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7091-b857-341edf28278a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7091-b857-341edf28278a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-74a6-b27f-767147bbdc1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7091-b857-341edf28278a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-74a6-b27f-767147bbdc1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7331-8553-dfcdb9585055" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-74a6-b27f-767147bbdc1d" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7331-8553-dfcdb9585055" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e4114-b67f-796e-bb4b-f3f9501699ce" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7bb7-830b-04550edbf89e" xlink:to="loc_srt_RangeAxis_019e4114-b67f-796e-bb4b-f3f9501699ce" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-796e-bb4b-f3f9501699ce_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e4114-b67f-796e-bb4b-f3f9501699ce" xlink:to="loc_srt_RangeMember_019e4114-b67f-796e-bb4b-f3f9501699ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-7654-ad06-8c0416605f47" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e4114-b67f-796e-bb4b-f3f9501699ce" xlink:to="loc_srt_RangeMember_019e4114-b67f-7654-ad06-8c0416605f47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e4114-b67f-7a38-a21d-f98766d4e4d8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7654-ad06-8c0416605f47" xlink:to="loc_srt_MinimumMember_019e4114-b67f-7a38-a21d-f98766d4e4d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e4114-b67f-7fc1-b5fe-b8992760e209" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7654-ad06-8c0416605f47" xlink:to="loc_srt_MaximumMember_019e4114-b67f-7fc1-b5fe-b8992760e209" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_019e4114-b67f-7d14-9bdb-6b5fb52702a7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7654-ad06-8c0416605f47" xlink:to="loc_srt_WeightedAverageMember_019e4114-b67f-7d14-9bdb-6b5fb52702a7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_019e4114-b67f-7cae-b2f2-d7d4ca641961" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7bb7-830b-04550edbf89e" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_019e4114-b67f-7cae-b2f2-d7d4ca641961" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7cae-b2f2-d7d4ca641961_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019e4114-b67f-7cae-b2f2-d7d4ca641961" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7cae-b2f2-d7d4ca641961_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7e54-8da1-3b5b7cce69d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019e4114-b67f-7cae-b2f2-d7d4ca641961" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7e54-8da1-3b5b7cce69d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputDiscountRateMember_019e4114-b67f-7128-b43c-1e36e7877593" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputDiscountRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7e54-8da1-3b5b7cce69d1" xlink:to="loc_us-gaap_MeasurementInputDiscountRateMember_019e4114-b67f-7128-b43c-1e36e7877593" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputEntityCreditRiskMember_019e4114-b67f-759b-9ce1-76043b8cd500" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputEntityCreditRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7e54-8da1-3b5b7cce69d1" xlink:to="loc_us-gaap_MeasurementInputEntityCreditRiskMember_019e4114-b67f-759b-9ce1-76043b8cd500" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPrepaymentRateMember_019e4114-b67f-7106-9a7a-1717f84ddabb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputPrepaymentRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019e4114-b67f-7e54-8da1-3b5b7cce69d1" xlink:to="loc_us-gaap_MeasurementInputPrepaymentRateMember_019e4114-b67f-7106-9a7a-1717f84ddabb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansReceivableMeasurementInput_019e4114-b67f-778a-95ce-7ae5b7686af7" xlink:href="upst-20260630.xsd#upst_LoansReceivableMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_LoansReceivableMeasurementInput_019e4114-b67f-778a-95ce-7ae5b7686af7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SecuritizedLoansMeasurementInput_019e4114-b67f-7df1-b0cf-47d4c3e76256" xlink:href="upst-20260630.xsd#upst_SecuritizedLoansMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_SecuritizedLoansMeasurementInput_019e4114-b67f-7df1-b0cf-47d4c3e76256" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditReceivableMeasurementInput_019e4114-b67f-743d-ac0b-f1db1a2b59b9" xlink:href="upst-20260630.xsd#upst_LineOfCreditReceivableMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_LineOfCreditReceivableMeasurementInput_019e4114-b67f-743d-ac0b-f1db1a2b59b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_NotesReceivableMeasurementInput_019e4114-b67f-791e-a1fa-374e2a9cdfec" xlink:href="upst-20260630.xsd#upst_NotesReceivableMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_NotesReceivableMeasurementInput_019e4114-b67f-791e-a1fa-374e2a9cdfec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PayablesToSecuritizationNoteHoldersMeasurementInput_019e4114-b67f-757e-b820-657fc7ad3d80" xlink:href="upst-20260630.xsd#upst_PayablesToSecuritizationNoteHoldersMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_PayablesToSecuritizationNoteHoldersMeasurementInput_019e4114-b67f-757e-b820-657fc7ad3d80" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestAssetMeasurementInput_019e4114-b67f-7e07-8b11-6b4f0a979536" xlink:href="upst-20260630.xsd#upst_BeneficialInterestAssetMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_BeneficialInterestAssetMeasurementInput_019e4114-b67f-7e07-8b11-6b4f0a979536" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilityMeasurementInput_019e4114-b67f-77a6-915d-ba0ac38c58ca" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilityMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_BeneficialInterestLiabilityMeasurementInput_019e4114-b67f-77a6-915d-ba0ac38c58ca" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TrailingFeeLiabilityMeasurementInput_019e4114-b67f-7246-b8a9-0eff33c403b5" xlink:href="upst-20260630.xsd#upst_TrailingFeeLiabilityMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7925-9050-d97e3cc5003a" xlink:to="loc_upst_TrailingFeeLiabilityMeasurementInput_019e4114-b67f-7246-b8a9-0eff33c403b5" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofSensitivityAnalysisofFairValueDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofSensitivityAnalysisofFairValueDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofSensitivityAnalysisofFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7496-80ca-a0714b8dce51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7496-80ca-a0714b8dce51" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7639-9171-7bf10ed3e37c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7496-80ca-a0714b8dce51" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7639-9171-7bf10ed3e37c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7639-9171-7bf10ed3e37c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7639-9171-7bf10ed3e37c" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7639-9171-7bf10ed3e37c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7ec4-b3d1-89a8d861d601" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7639-9171-7bf10ed3e37c" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7ec4-b3d1-89a8d861d601" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7bdb-9c51-7cff17d05d56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7ec4-b3d1-89a8d861d601" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7bdb-9c51-7cff17d05d56" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7de6-8bdb-150568a2ffa8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7496-80ca-a0714b8dce51" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7de6-8bdb-150568a2ffa8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7de6-8bdb-150568a2ffa8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7de6-8bdb-150568a2ffa8" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7de6-8bdb-150568a2ffa8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-71f4-a81f-661162d3e96e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7de6-8bdb-150568a2ffa8" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-71f4-a81f-661162d3e96e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansHeldForSaleAndInvestmentMember_019e4114-b67f-7b34-a32c-988195c84e44" xlink:href="upst-20260630.xsd#upst_LoansHeldForSaleAndInvestmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-71f4-a81f-661162d3e96e" xlink:to="loc_upst_LoansHeldForSaleAndInvestmentMember_019e4114-b67f-7b34-a32c-988195c84e44" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestsMember_019e4114-b67f-70b4-b127-63e0eb8918d8" xlink:href="upst-20260630.xsd#upst_BeneficialInterestsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-71f4-a81f-661162d3e96e" xlink:to="loc_upst_BeneficialInterestsMember_019e4114-b67f-70b4-b127-63e0eb8918d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7eeb-8a5c-e48a968d2486" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7496-80ca-a0714b8dce51" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7eeb-8a5c-e48a968d2486" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7eeb-8a5c-e48a968d2486_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7eeb-8a5c-e48a968d2486" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7eeb-8a5c-e48a968d2486_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7e63-a891-59ffb39786f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7eeb-8a5c-e48a968d2486" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7e63-a891-59ffb39786f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestsMember_019e4114-b67f-7582-94e9-4a3e5fa179dd" xlink:href="upst-20260630.xsd#upst_BeneficialInterestsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7e63-a891-59ffb39786f3" xlink:to="loc_upst_BeneficialInterestsMember_019e4114-b67f-7582-94e9-4a3e5fa179dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableFairValueDisclosure_019e4114-b67f-7765-a3be-4a47750dbe85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_us-gaap_LoansReceivableFairValueDisclosure_019e4114-b67f-7765-a3be-4a47750dbe85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestAsset_019e4114-b67f-7c32-b938-a161ba96fd7f" xlink:href="upst-20260630.xsd#upst_BeneficialInterestAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_upst_BeneficialInterestAsset_019e4114-b67f-7c32-b938-a161ba96fd7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DiscountRateAbstract_019e4114-b67f-7a64-bae1-15dc136f98fb" xlink:href="upst-20260630.xsd#upst_DiscountRateAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_upst_DiscountRateAbstract_019e4114-b67f-7a64-bae1-15dc136f98fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf100BasisPointIncreaseInDiscountRates_019e4114-b67f-76fd-bd2d-31b4f030291a" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueAssetsImpactOf100BasisPointIncreaseInDiscountRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_DiscountRateAbstract_019e4114-b67f-7a64-bae1-15dc136f98fb" xlink:to="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf100BasisPointIncreaseInDiscountRates_019e4114-b67f-76fd-bd2d-31b4f030291a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf200BasisPointIncreaseInDiscountRates_019e4114-b67f-79b9-a3ac-6bbda59a7a44" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueAssetsImpactOf200BasisPointIncreaseInDiscountRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_DiscountRateAbstract_019e4114-b67f-7a64-bae1-15dc136f98fb" xlink:to="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf200BasisPointIncreaseInDiscountRates_019e4114-b67f-79b9-a3ac-6bbda59a7a44" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ExpectedCreditLossRateAbstract_019e4114-b67f-7559-8d30-7f587bca2843" xlink:href="upst-20260630.xsd#upst_ExpectedCreditLossRateAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_upst_ExpectedCreditLossRateAbstract_019e4114-b67f-7559-8d30-7f587bca2843" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf10PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-797a-8258-f23c77654893" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueAssetsImpactOf10PercentAdverseChangeInExpectedCreditLossRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedCreditLossRateAbstract_019e4114-b67f-7559-8d30-7f587bca2843" xlink:to="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf10PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-797a-8258-f23c77654893" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf20PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-7aff-9b57-add8b4d7cf4d" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueAssetsImpactOf20PercentAdverseChangeInExpectedCreditLossRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedCreditLossRateAbstract_019e4114-b67f-7559-8d30-7f587bca2843" xlink:to="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf20PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-7aff-9b57-add8b4d7cf4d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ExpectedPrepaymentRatesAbstract_019e4114-b67f-7bb7-aff2-7243c65c3613" xlink:href="upst-20260630.xsd#upst_ExpectedPrepaymentRatesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_upst_ExpectedPrepaymentRatesAbstract_019e4114-b67f-7bb7-aff2-7243c65c3613" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf10PercentAdverseChangeInExpectedPrepaymentRates_019e4114-b67f-7f97-bfa5-f68bbf3a42e3" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueAssetsImpactOf10PercentAdverseChangeInExpectedPrepaymentRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedPrepaymentRatesAbstract_019e4114-b67f-7bb7-aff2-7243c65c3613" xlink:to="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf10PercentAdverseChangeInExpectedPrepaymentRates_019e4114-b67f-7f97-bfa5-f68bbf3a42e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf20PercentAdverseChangeInExpectedPrepaymentRates_019e4114-b67f-7830-af62-64364b657a95" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueAssetsImpactOf20PercentAdverseChangeInExpectedPrepaymentRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedPrepaymentRatesAbstract_019e4114-b67f-7bb7-aff2-7243c65c3613" xlink:to="loc_upst_SensitivityAnalysisOfFairValueAssetsImpactOf20PercentAdverseChangeInExpectedPrepaymentRates_019e4114-b67f-7830-af62-64364b657a95" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilities_019e4114-b67f-7654-92b7-595c4b94c9a2" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedPrepaymentRatesAbstract_019e4114-b67f-7bb7-aff2-7243c65c3613" xlink:to="loc_upst_BeneficialInterestLiabilities_019e4114-b67f-7654-92b7-595c4b94c9a2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ExpectedCreditLossRateLiabilitiesAbstract_019e4114-b67f-7a01-b57e-46c3783c1cfc" xlink:href="upst-20260630.xsd#upst_ExpectedCreditLossRateLiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7750-8623-3a9b3a1f972d" xlink:to="loc_upst_ExpectedCreditLossRateLiabilitiesAbstract_019e4114-b67f-7a01-b57e-46c3783c1cfc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueLiabilitiesImpactOf10PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-7c65-83ca-da6cbc3d5ccc" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueLiabilitiesImpactOf10PercentAdverseChangeInExpectedCreditLossRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedCreditLossRateLiabilitiesAbstract_019e4114-b67f-7a01-b57e-46c3783c1cfc" xlink:to="loc_upst_SensitivityAnalysisOfFairValueLiabilitiesImpactOf10PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-7c65-83ca-da6cbc3d5ccc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueLiabilitiesImpactOf20PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-7523-8c68-71971eec1bb7" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueLiabilitiesImpactOf20PercentAdverseChangeInExpectedCreditLossRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ExpectedCreditLossRateLiabilitiesAbstract_019e4114-b67f-7a01-b57e-46c3783c1cfc" xlink:to="loc_upst_SensitivityAnalysisOfFairValueLiabilitiesImpactOf20PercentAdverseChangeInExpectedCreditLossRates_019e4114-b67f-7523-8c68-71971eec1bb7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofRollforwardofLevel3AssetsDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofRollforwardofLevel3AssetsDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofRollforwardofLevel3AssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-7bd0-89b7-5f8b9be34976" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7cf1-92fc-13afd7917243" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-7bd0-89b7-5f8b9be34976" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7cf1-92fc-13afd7917243" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-753c-8ad8-77d84e2f3a9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7cf1-92fc-13afd7917243" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-753c-8ad8-77d84e2f3a9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-753c-8ad8-77d84e2f3a9c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-753c-8ad8-77d84e2f3a9c" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-753c-8ad8-77d84e2f3a9c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-724f-b853-23a2a335f921" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-753c-8ad8-77d84e2f3a9c" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-724f-b853-23a2a335f921" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableMember_019e4114-b67f-7b0d-b840-fe9f2282d2be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-724f-b853-23a2a335f921" xlink:to="loc_us-gaap_LoansReceivableMember_019e4114-b67f-7b0d-b840-fe9f2282d2be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoanHeldForSaleMember_019e4114-b67f-7408-93d4-411bd9fec2fe" xlink:href="upst-20260630.xsd#upst_LoanHeldForSaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansReceivableMember_019e4114-b67f-7b0d-b840-fe9f2282d2be" xlink:to="loc_upst_LoanHeldForSaleMember_019e4114-b67f-7408-93d4-411bd9fec2fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansHeldForInvestmentMember_019e4114-b67f-7f39-9eb8-7c11d044b82e" xlink:href="upst-20260630.xsd#upst_LoansHeldForInvestmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansReceivableMember_019e4114-b67f-7b0d-b840-fe9f2282d2be" xlink:to="loc_upst_LoansHeldForInvestmentMember_019e4114-b67f-7f39-9eb8-7c11d044b82e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoansHeldForSaleInConsolidatedSecuritizationMember_019e4114-b67f-7e7a-bb91-31d5c13bbb01" xlink:href="upst-20260630.xsd#upst_LoansHeldForSaleInConsolidatedSecuritizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansReceivableMember_019e4114-b67f-7b0d-b840-fe9f2282d2be" xlink:to="loc_upst_LoansHeldForSaleInConsolidatedSecuritizationMember_019e4114-b67f-7e7a-bb91-31d5c13bbb01" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76f6-93bf-267ff4a1ca0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7cf1-92fc-13afd7917243" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76f6-93bf-267ff4a1ca0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-76f6-93bf-267ff4a1ca0d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76f6-93bf-267ff4a1ca0d" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-76f6-93bf-267ff4a1ca0d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e16-b32e-82768d32a357" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76f6-93bf-267ff4a1ca0d" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e16-b32e-82768d32a357" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7daa-86c0-6f39ef6a7ec1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e16-b32e-82768d32a357" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7daa-86c0-6f39ef6a7ec1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-7bd0-89b7-5f8b9be34976" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019e4114-b67f-7b2f-b043-2505f0c0b356" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019e4114-b67f-7b2f-b043-2505f0c0b356" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetReclassificationOfLoans_019e4114-b67f-7f45-a360-1cd74df57b2c" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetReclassificationOfLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetReclassificationOfLoans_019e4114-b67f-7f45-a360-1cd74df57b2c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases_019e4114-b67f-7279-8df6-9d2aba3f413c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases_019e4114-b67f-7279-8df6-9d2aba3f413c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales_019e4114-b67f-75a3-b5c2-1bd6a472f0f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales_019e4114-b67f-75a3-b5c2-1bd6a472f0f8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchaseOfLoansForImmediateResaleToInvestors_019e4114-b67f-785b-ac84-cbc82376b6dd" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchaseOfLoansForImmediateResaleToInvestors"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchaseOfLoansForImmediateResaleToInvestors_019e4114-b67f-785b-ac84-cbc82376b6dd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetImmediateResaleToInvestors_019e4114-b67f-79f4-ae9f-63275198a7e2" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetImmediateResaleToInvestors"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetImmediateResaleToInvestors_019e4114-b67f-79f4-ae9f-63275198a7e2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetRepaymentsReceived_019e4114-b67f-754f-94c6-78bc742fb392" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetRepaymentsReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetRepaymentsReceived_019e4114-b67f-754f-94c6-78bc742fb392" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019e4114-b67f-7bdf-baf6-1880c05f31de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019e4114-b67f-7bdf-baf6-1880c05f31de" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOtherIncreaseDecrease_019e4114-b67f-7479-a81e-9d8de03bdc0d" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOtherIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-7d96-aadf-00793ae332ca" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOtherIncreaseDecrease_019e4114-b67f-7479-a81e-9d8de03bdc0d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019e4114-b67f-7b77-8798-c8052257137c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofAggregateFairValueandPrincipalOutstandingofAllLoansAndLoans90DaysorMorePastDueDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofAggregateFairValueandPrincipalOutstandingofAllLoansAndLoans90DaysorMorePastDueDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofAggregateFairValueandPrincipalOutstandingofAllLoansAndLoans90DaysorMorePastDueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019e4114-b67f-724a-9587-1b9e248ae2d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019e4114-b67f-782a-b29c-673ed34d972a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019e4114-b67f-724a-9587-1b9e248ae2d4" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019e4114-b67f-782a-b29c-673ed34d972a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019e4114-b67f-7636-a167-d53bd23c7f0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019e4114-b67f-782a-b29c-673ed34d972a" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019e4114-b67f-7636-a167-d53bd23c7f0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019e4114-b67f-7636-a167-d53bd23c7f0a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019e4114-b67f-7636-a167-d53bd23c7f0a" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019e4114-b67f-7636-a167-d53bd23c7f0a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019e4114-b67f-78a4-a11d-2c6510c5c185" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019e4114-b67f-7636-a167-d53bd23c7f0a" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019e4114-b67f-78a4-a11d-2c6510c5c185" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019e4114-b67f-7b3b-8121-9774544b1f24" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019e4114-b67f-78a4-a11d-2c6510c5c185" xlink:to="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019e4114-b67f-7b3b-8121-9774544b1f24" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b67f-7d15-9609-085128c86a52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019e4114-b67f-782a-b29c-673ed34d972a" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b67f-7d15-9609-085128c86a52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019e4114-b67f-7d15-9609-085128c86a52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b67f-7d15-9609-085128c86a52" xlink:to="loc_us-gaap_ReceivableTypeDomain_019e4114-b67f-7d15-9609-085128c86a52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019e4114-b67f-7d23-98c0-69082af20b94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e4114-b67f-7d15-9609-085128c86a52" xlink:to="loc_us-gaap_ReceivableTypeDomain_019e4114-b67f-7d23-98c0-69082af20b94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableMember_019e4114-b67f-764e-af90-0e4519b45b5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019e4114-b67f-7d23-98c0-69082af20b94" xlink:to="loc_us-gaap_LoansReceivableMember_019e4114-b67f-764e-af90-0e4519b45b5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7c6a-af82-26473c48b9f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019e4114-b67f-782a-b29c-673ed34d972a" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7c6a-af82-26473c48b9f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7c6a-af82-26473c48b9f0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7c6a-af82-26473c48b9f0" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7c6a-af82-26473c48b9f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7e91-ac2a-6e7631d521b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7c6a-af82-26473c48b9f0" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7e91-ac2a-6e7631d521b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AutoLoansMember_019e4114-b67f-74c7-bc13-94caeaf1b62a" xlink:href="upst-20260630.xsd#upst_AutoLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7e91-ac2a-6e7631d521b6" xlink:to="loc_upst_AutoLoansMember_019e4114-b67f-74c7-bc13-94caeaf1b62a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HomeEquityMember_019e4114-b67f-7f1e-b3ef-639b38e50f34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HomeEquityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7e91-ac2a-6e7631d521b6" xlink:to="loc_us-gaap_HomeEquityMember_019e4114-b67f-7f1e-b3ef-639b38e50f34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_019e4114-b67f-72dc-a4ac-8560330793a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019e4114-b67f-724a-9587-1b9e248ae2d4" xlink:to="loc_us-gaap_NotesReceivableGross_019e4114-b67f-72dc-a4ac-8560330793a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019e4114-b67f-7ac6-bbb2-0d0e4ac19e1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019e4114-b67f-724a-9587-1b9e248ae2d4" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019e4114-b67f-7ac6-bbb2-0d0e4ac19e1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableNet_019e4114-b67f-78a2-b855-501df24209d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019e4114-b67f-724a-9587-1b9e248ae2d4" xlink:to="loc_us-gaap_NotesReceivableNet_019e4114-b67f-78a2-b855-501df24209d1" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7d40-bf2b-2d1712b7fcfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7d40-bf2b-2d1712b7fcfc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7a10-8169-7aa35fa9e441" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e4114-b67f-7d40-bf2b-2d1712b7fcfc" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7a10-8169-7aa35fa9e441" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7a10-8169-7aa35fa9e441_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7a10-8169-7aa35fa9e441" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7a10-8169-7aa35fa9e441_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7b98-9104-6d90802afb37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7a10-8169-7aa35fa9e441" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7b98-9104-6d90802afb37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7479-9fab-7cbfee7bd747" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7b98-9104-6d90802afb37" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7479-9fab-7cbfee7bd747" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditReceivableFairValueDisclosure_019e4114-b67f-78fb-9416-f6bc367dc95f" xlink:href="upst-20260630.xsd#upst_LineOfCreditReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_upst_LineOfCreditReceivableFairValueDisclosure_019e4114-b67f-78fb-9416-f6bc367dc95f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableFairValueDisclosure_019e4114-b67f-7630-8e92-3fc9c28ac91f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_us-gaap_NotesReceivableFairValueDisclosure_019e4114-b67f-7630-8e92-3fc9c28ac91f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PayableToSecuritizationNoteHoldersAtFairValue_019e4114-b67f-7bf0-922b-1c0103e28bd3" xlink:href="upst-20260630.xsd#upst_PayableToSecuritizationNoteHoldersAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_upst_PayableToSecuritizationNoteHoldersAtFairValue_019e4114-b67f-7bf0-922b-1c0103e28bd3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-7166-800a-0ac1ce62878c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-7166-800a-0ac1ce62878c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-7f71-b1af-a2b1735184d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilityAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-7f71-b1af-a2b1735184d9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestAsset_019e4114-b67f-7158-a8e8-130685110d59" xlink:href="upst-20260630.xsd#upst_BeneficialInterestAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_upst_BeneficialInterestAsset_019e4114-b67f-7158-a8e8-130685110d59" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilities_019e4114-b67f-73d0-bea6-b1ea2a6c350a" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_upst_BeneficialInterestLiabilities_019e4114-b67f-73d0-bea6-b1ea2a6c350a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TrailingFeeLiabilityAtFairValueAmount_019e4114-b67f-7cf9-8b1c-6df00cd00ab1" xlink:href="upst-20260630.xsd#upst_TrailingFeeLiabilityAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e4114-b67f-7e38-9555-f8d54accde67" xlink:to="loc_upst_TrailingFeeLiabilityAtFairValueAmount_019e4114-b67f-7cf9-8b1c-6df00cd00ab1" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofLevelThreeAssetsandLiabilitiesRollforwardDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofLevelThreeAssetsandLiabilitiesRollforwardDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofLevelThreeAssetsandLiabilitiesRollforwardDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-7466-ab40-b93b692ecd23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7c51-8233-d7d2d5a367f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-7466-ab40-b93b692ecd23" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7c51-8233-d7d2d5a367f2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e94-8547-4946954c1910" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7c51-8233-d7d2d5a367f2" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e94-8547-4946954c1910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e94-8547-4946954c1910_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e94-8547-4946954c1910" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e94-8547-4946954c1910_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7fbf-856d-f9e54d96f367" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e94-8547-4946954c1910" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7fbf-856d-f9e54d96f367" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-76a0-9aa2-bac419957827" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7fbf-856d-f9e54d96f367" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-76a0-9aa2-bac419957827" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7066-bd47-d3de2d6d5a62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-7c51-8233-d7d2d5a367f2" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7066-bd47-d3de2d6d5a62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7066-bd47-d3de2d6d5a62_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7066-bd47-d3de2d6d5a62" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7066-bd47-d3de2d6d5a62_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-70e3-9319-751c9d7780ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-7066-bd47-d3de2d6d5a62" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-70e3-9319-751c9d7780ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditReceivableMember_019e4114-b67f-7344-9c83-0ef8da452db9" xlink:href="upst-20260630.xsd#upst_LineOfCreditReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-70e3-9319-751c9d7780ea" xlink:to="loc_upst_LineOfCreditReceivableMember_019e4114-b67f-7344-9c83-0ef8da452db9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_NotesReceivableAndResidualCertificatesMember_019e4114-b67f-762d-a311-b0e0d0b581cd" xlink:href="upst-20260630.xsd#upst_NotesReceivableAndResidualCertificatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-70e3-9319-751c9d7780ea" xlink:to="loc_upst_NotesReceivableAndResidualCertificatesMember_019e4114-b67f-762d-a311-b0e0d0b581cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestsMember_019e4114-b67f-7244-834b-8841f9cd3127" xlink:href="upst-20260630.xsd#upst_BeneficialInterestsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-70e3-9319-751c9d7780ea" xlink:to="loc_upst_BeneficialInterestsMember_019e4114-b67f-7244-834b-8841f9cd3127" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-7466-ab40-b93b692ecd23" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019e4114-b67f-774f-a534-b14b908e94b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019e4114-b67f-774f-a534-b14b908e94b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAdditions_019e4114-b67f-77e2-80d8-4df20d814d05" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAdditions_019e4114-b67f-77e2-80d8-4df20d814d05" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues_019e4114-b67f-7804-96ea-14f4871b0e40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues_019e4114-b67f-7804-96ea-14f4871b0e40" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements_019e4114-b67f-7f27-9a56-eb94bb065e81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements_019e4114-b67f-7f27-9a56-eb94bb065e81" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019e4114-b67f-7802-9751-8cfca08e0cf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019e4114-b67f-7802-9751-8cfca08e0cf6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetChangesInAccruedInterest_019e4114-b67f-7fe4-b104-b580acbfe374" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetChangesInAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-71bd-ba2e-c999bbc192d0" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetChangesInAccruedInterest_019e4114-b67f-7fe4-b104-b580acbfe374" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019e4114-b67f-7a92-a537-a7e28b77ff5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-763a-8c6f-42f94d241e26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-714b-8e1a-384d16f78d78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-763a-8c6f-42f94d241e26" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-714b-8e1a-384d16f78d78" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-721b-82c0-19f7545b6039" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-714b-8e1a-384d16f78d78" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-721b-82c0-19f7545b6039" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-721b-82c0-19f7545b6039_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7226-8f31-61716c377ea1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-769a-82c5-bb12bc0c0534" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7894-ad90-eb1474380ef8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019e4114-b67f-714b-8e1a-384d16f78d78" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7894-ad90-eb1474380ef8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7894-ad90-eb1474380ef8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7894-ad90-eb1474380ef8" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7894-ad90-eb1474380ef8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-70a8-91f1-179ff28db90a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7894-ad90-eb1474380ef8" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-70a8-91f1-179ff28db90a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PayablesToSecuritizationNoteHoldersMember_019e4114-b67f-74ef-8011-2366348edae4" xlink:href="upst-20260630.xsd#upst_PayablesToSecuritizationNoteHoldersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-70a8-91f1-179ff28db90a" xlink:to="loc_upst_PayablesToSecuritizationNoteHoldersMember_019e4114-b67f-74ef-8011-2366348edae4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BeneficialInterestLiabilitiesMember_019e4114-b67f-75e5-a1e4-470e4a936f63" xlink:href="upst-20260630.xsd#upst_BeneficialInterestLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-70a8-91f1-179ff28db90a" xlink:to="loc_upst_BeneficialInterestLiabilitiesMember_019e4114-b67f-75e5-a1e4-470e4a936f63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TrailingFeeLiabilitiesMember_019e4114-b67f-7b3d-b8b1-fa6aae61a50c" xlink:href="upst-20260630.xsd#upst_TrailingFeeLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-70a8-91f1-179ff28db90a" xlink:to="loc_upst_TrailingFeeLiabilitiesMember_019e4114-b67f-7b3d-b8b1-fa6aae61a50c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019e4114-b67f-763a-8c6f-42f94d241e26" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019e4114-b67f-7ec7-9518-5ca1fbddda00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019e4114-b67f-7ec7-9518-5ca1fbddda00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityAdditions_019e4114-b67f-7804-8703-29159085349d" xlink:href="upst-20260630.xsd#upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:to="loc_upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityAdditions_019e4114-b67f-7804-8703-29159085349d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues_019e4114-b67f-7bf3-9b74-0d65cb41bca7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues_019e4114-b67f-7bf3-9b74-0d65cb41bca7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_019e4114-b67f-70da-b374-f037f1a2eee2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_019e4114-b67f-70da-b374-f037f1a2eee2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_019e4114-b67f-709b-9eae-6c1516db20d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019e4114-b67f-790b-a42e-8cc9fe1cc590" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_019e4114-b67f-709b-9eae-6c1516db20d3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019e4114-b67f-7088-a656-757c38dc6632" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofLevel3FairValueAssumptionsforLoanServicingAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofLevel3FairValueAssumptionsforLoanServicingAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofLevel3FairValueAssumptionsforLoanServicingAssetsandLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems_019e4114-b67f-7e9d-9550-77a219c509af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7c12-b743-58a44a3fe603" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems_019e4114-b67f-7e9d-9550-77a219c509af" xlink:to="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7c12-b743-58a44a3fe603" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e4114-b67f-7ec5-9032-a880b42bf46f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7c12-b743-58a44a3fe603" xlink:to="loc_srt_RangeAxis_019e4114-b67f-7ec5-9032-a880b42bf46f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-7ec5-9032-a880b42bf46f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e4114-b67f-7ec5-9032-a880b42bf46f" xlink:to="loc_srt_RangeMember_019e4114-b67f-7ec5-9032-a880b42bf46f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-7980-b4a8-e0aa7295a7a6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e4114-b67f-7ec5-9032-a880b42bf46f" xlink:to="loc_srt_RangeMember_019e4114-b67f-7980-b4a8-e0aa7295a7a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e4114-b67f-711f-85c3-f1145301c895" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7980-b4a8-e0aa7295a7a6" xlink:to="loc_srt_MinimumMember_019e4114-b67f-711f-85c3-f1145301c895" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e4114-b67f-7ba1-8c93-5c4942dac89c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7980-b4a8-e0aa7295a7a6" xlink:to="loc_srt_MaximumMember_019e4114-b67f-7ba1-8c93-5c4942dac89c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_019e4114-b67f-747f-9e21-7e1543f8b72e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7980-b4a8-e0aa7295a7a6" xlink:to="loc_srt_WeightedAverageMember_019e4114-b67f-747f-9e21-7e1543f8b72e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e2d-ad67-7fd20b5d4bcc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7c12-b743-58a44a3fe603" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e2d-ad67-7fd20b5d4bcc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e2d-ad67-7fd20b5d4bcc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e2d-ad67-7fd20b5d4bcc" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e2d-ad67-7fd20b5d4bcc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7b36-a932-c221effcf63b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-7e2d-ad67-7fd20b5d4bcc" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7b36-a932-c221effcf63b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7def-bdac-22de470175d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7b36-a932-c221effcf63b" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7def-bdac-22de470175d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-71f9-8aa1-3361c3ae09a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7c12-b743-58a44a3fe603" xlink:to="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-71f9-8aa1-3361c3ae09a1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-71f9-8aa1-3361c3ae09a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-71f9-8aa1-3361c3ae09a1" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-71f9-8aa1-3361c3ae09a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-72df-a0e8-2638bbee3950" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019e4114-b67f-71f9-8aa1-3361c3ae09a1" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-72df-a0e8-2638bbee3950" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDiscountedCashFlowMember_019e4114-b67f-7782-b481-d6518c76a5d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDiscountedCashFlowMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_019e4114-b67f-72df-a0e8-2638bbee3950" xlink:to="loc_us-gaap_ValuationTechniqueDiscountedCashFlowMember_019e4114-b67f-7782-b481-d6518c76a5d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-71cb-ab28-5bfc2f61be46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7c12-b743-58a44a3fe603" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-71cb-ab28-5bfc2f61be46" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-71cb-ab28-5bfc2f61be46_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-71cb-ab28-5bfc2f61be46" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-71cb-ab28-5bfc2f61be46_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7458-99af-721e767515c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-71cb-ab28-5bfc2f61be46" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7458-99af-721e767515c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AutoLoansMember_019e4114-b67f-71fe-9ce4-c80a3b747f8c" xlink:href="upst-20260630.xsd#upst_AutoLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7458-99af-721e767515c8" xlink:to="loc_upst_AutoLoansMember_019e4114-b67f-71fe-9ce4-c80a3b747f8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_PersonalLoansMember_019e4114-b67f-794c-89d4-a52aeb023ac4" xlink:href="upst-20260630.xsd#upst_PersonalLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7458-99af-721e767515c8" xlink:to="loc_upst_PersonalLoansMember_019e4114-b67f-794c-89d4-a52aeb023ac4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesDiscountRate_019e4114-b67f-7af3-8911-a3f90464e84e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems_019e4114-b67f-7e9d-9550-77a219c509af" xlink:to="loc_us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesDiscountRate_019e4114-b67f-7af3-8911-a3f90464e84e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesExpectedCreditLosses_019e4114-b67f-790b-a264-43cd2f1fc197" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesExpectedCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems_019e4114-b67f-7e9d-9550-77a219c509af" xlink:to="loc_us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesExpectedCreditLosses_019e4114-b67f-790b-a264-43cd2f1fc197" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AssumptionForFairValueOfAssetsOrLiabilitiesThatRelateToTransferorsContinuingInvolvementMarketServicingRate_019e4114-b67f-7f59-a135-3a06a6b70055" xlink:href="upst-20260630.xsd#upst_AssumptionForFairValueOfAssetsOrLiabilitiesThatRelateToTransferorsContinuingInvolvementMarketServicingRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems_019e4114-b67f-7e9d-9550-77a219c509af" xlink:to="loc_upst_AssumptionForFairValueOfAssetsOrLiabilitiesThatRelateToTransferorsContinuingInvolvementMarketServicingRate_019e4114-b67f-7f59-a135-3a06a6b70055" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesPrepaymentSpeed_019e4114-b67f-7133-bc79-3955285b79b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesPrepaymentSpeed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems_019e4114-b67f-7e9d-9550-77a219c509af" xlink:to="loc_us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesPrepaymentSpeed_019e4114-b67f-7133-bc79-3955285b79b2" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofFairValueSensitivityofLoanServicingAssetsandLiabilitiestoAdverseChangesinKeyAssumptionsDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofFairValueSensitivityofLoanServicingAssetsandLiabilitiestoAdverseChangesinKeyAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofFairValueSensitivityofLoanServicingAssetsandLiabilitiestoAdverseChangesinKeyAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionLineItems_019e4114-b67f-76e2-afcf-361589e7ba6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7f03-9a20-7d5df155d53e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionLineItems_019e4114-b67f-76e2-afcf-361589e7ba6e" xlink:to="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7f03-9a20-7d5df155d53e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76a4-823e-bab03ca875fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7f03-9a20-7d5df155d53e" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76a4-823e-bab03ca875fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-76a4-823e-bab03ca875fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76a4-823e-bab03ca875fd" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-76a4-823e-bab03ca875fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-71d5-9f99-70c5af4d1479" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-76a4-823e-bab03ca875fd" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-71d5-9f99-70c5af4d1479" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-722d-a426-85308963f235" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-71d5-9f99-70c5af4d1479" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-722d-a426-85308963f235" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-72de-9d1a-b963b3115667" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesTable_019e4114-b67f-7f03-9a20-7d5df155d53e" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-72de-9d1a-b963b3115667" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-72de-9d1a-b963b3115667_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-72de-9d1a-b963b3115667" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-72de-9d1a-b963b3115667_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7707-baaf-1278298d00c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-72de-9d1a-b963b3115667" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7707-baaf-1278298d00c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoanServicingAssetsMember_019e4114-b67f-79d1-b367-dd28563aa0c5" xlink:href="upst-20260630.xsd#upst_LoanServicingAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-7707-baaf-1278298d00c6" xlink:to="loc_upst_LoanServicingAssetsMember_019e4114-b67f-79d1-b367-dd28563aa0c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-7eba-88f4-d59401d1bd5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionLineItems_019e4114-b67f-76e2-afcf-361589e7ba6e" xlink:to="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-7eba-88f4-d59401d1bd5b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueTransferorsInterestsInTransferredFinancialAssetsImpactOf10PercentIncreaseInMarketServicingRates_019e4114-b67f-769b-b14c-02e51e1f8f17" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueTransferorsInterestsInTransferredFinancialAssetsImpactOf10PercentIncreaseInMarketServicingRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionLineItems_019e4114-b67f-76e2-afcf-361589e7ba6e" xlink:to="loc_upst_SensitivityAnalysisOfFairValueTransferorsInterestsInTransferredFinancialAssetsImpactOf10PercentIncreaseInMarketServicingRates_019e4114-b67f-769b-b14c-02e51e1f8f17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SensitivityAnalysisOfFairValueTransferorsInterestsInTransferredFinancialAssetsImpactOf20PercentIncreaseInMarketServicingRates_019e4114-b67f-702b-8f6c-8726f2ef71cf" xlink:href="upst-20260630.xsd#upst_SensitivityAnalysisOfFairValueTransferorsInterestsInTransferredFinancialAssetsImpactOf20PercentIncreaseInMarketServicingRates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfAdverseChangeInAssumptionLineItems_019e4114-b67f-76e2-afcf-361589e7ba6e" xlink:to="loc_upst_SensitivityAnalysisOfFairValueTransferorsInterestsInTransferredFinancialAssetsImpactOf20PercentIncreaseInMarketServicingRates_019e4114-b67f-702b-8f6c-8726f2ef71cf" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofServicingLiabilitiesatFairValueRollforwardDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#FairValueMeasurementScheduleofServicingLiabilitiesatFairValueRollforwardDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/FairValueMeasurementScheduleofServicingLiabilitiesatFairValueRollforwardDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetsAtFairValueLineItems_019e4114-b67f-71a6-b05b-67f52772e7d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetsAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetsAtFairValueTable_019e4114-b67f-797a-8adf-55dc2a983ea6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetsAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ServicingAssetsAtFairValueLineItems_019e4114-b67f-71a6-b05b-67f52772e7d9" xlink:to="loc_us-gaap_ServicingAssetsAtFairValueTable_019e4114-b67f-797a-8adf-55dc2a983ea6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-782a-9734-07355d814776" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ServicingAssetsAtFairValueTable_019e4114-b67f-797a-8adf-55dc2a983ea6" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-782a-9734-07355d814776" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-782a-9734-07355d814776_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-782a-9734-07355d814776" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-782a-9734-07355d814776_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e9a-bda4-5f4dc409a8f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-782a-9734-07355d814776" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e9a-bda4-5f4dc409a8f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7047-b481-f933c8be0a4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-7e9a-bda4-5f4dc409a8f7" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-7047-b481-f933c8be0a4d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-78dd-85fa-e518d53bfd19" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ServicingAssetsAtFairValueTable_019e4114-b67f-797a-8adf-55dc2a983ea6" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-78dd-85fa-e518d53bfd19" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-78dd-85fa-e518d53bfd19_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-78dd-85fa-e518d53bfd19" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-78dd-85fa-e518d53bfd19_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-78e0-88cd-852a83121758" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e4114-b67f-78dd-85fa-e518d53bfd19" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-78e0-88cd-852a83121758" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoanServicingAssetsMember_019e4114-b67f-793b-b70e-3ad4a94b4fb6" xlink:href="upst-20260630.xsd#upst_LoanServicingAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e4114-b67f-78e0-88cd-852a83121758" xlink:to="loc_upst_LoanServicingAssetsMember_019e4114-b67f-793b-b70e-3ad4a94b4fb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAmountRollForward_019e4114-b67f-7446-b68b-348805924480" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAmountRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingAssetsAtFairValueLineItems_019e4114-b67f-71a6-b05b-67f52772e7d9" xlink:to="loc_us-gaap_ServicingAssetAtFairValueAmountRollForward_019e4114-b67f-7446-b68b-348805924480" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-73f8-aa03-cd18bfda10eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingAssetAtFairValueAmountRollForward_019e4114-b67f-7446-b68b-348805924480" xlink:to="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-73f8-aa03-cd18bfda10eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAdditions_019e4114-b67f-7d05-b827-7b77eaa2b37c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingAssetAtFairValueAmountRollForward_019e4114-b67f-7446-b68b-348805924480" xlink:to="loc_us-gaap_ServicingAssetAtFairValueAdditions_019e4114-b67f-7d05-b827-7b77eaa2b37c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ServicingAssetAtFairValueRepaymentsAndOtherChangesAtFairValueGainLossIncludedInEarnings_019e4114-b67f-7197-968d-e4ebca9d5ec1" xlink:href="upst-20260630.xsd#upst_ServicingAssetAtFairValueRepaymentsAndOtherChangesAtFairValueGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingAssetAtFairValueAmountRollForward_019e4114-b67f-7446-b68b-348805924480" xlink:to="loc_upst_ServicingAssetAtFairValueRepaymentsAndOtherChangesAtFairValueGainLossIncludedInEarnings_019e4114-b67f-7197-968d-e4ebca9d5ec1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingAssetAtFairValueAmount_019e4114-b67f-7af4-8993-f13b58179875" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingAssetAtFairValueAmount"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilitiesAtFairValueLineItems_019e4114-b67f-7db0-a121-f5061109f7c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilitiesAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilitiesAtFairValueTable_019e4114-b67f-7850-a313-052435644378" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilitiesAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ServicingLiabilitiesAtFairValueLineItems_019e4114-b67f-7db0-a121-f5061109f7c3" xlink:to="loc_us-gaap_ServicingLiabilitiesAtFairValueTable_019e4114-b67f-7850-a313-052435644378" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-75eb-b687-1ffaeb8a7797" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ServicingLiabilitiesAtFairValueTable_019e4114-b67f-7850-a313-052435644378" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4114-b67f-75eb-b687-1ffaeb8a7797" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-75eb-b687-1ffaeb8a7797_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4114-b67f-774a-a723-1877e7fe00b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e4114-b67f-704d-bc0c-97090c6e977c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7902-9856-5224de7caa88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ServicingLiabilitiesAtFairValueTable_019e4114-b67f-7850-a313-052435644378" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7902-9856-5224de7caa88" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7902-9856-5224de7caa88_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7902-9856-5224de7caa88" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7902-9856-5224de7caa88_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7130-9562-993e87b9cfa7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019e4114-b67f-7902-9856-5224de7caa88" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7130-9562-993e87b9cfa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LoanServicingLiabilitiesMember_019e4114-b67f-7940-9227-8d7c388a44cf" xlink:href="upst-20260630.xsd#upst_LoanServicingLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019e4114-b67f-7130-9562-993e87b9cfa7" xlink:to="loc_upst_LoanServicingLiabilitiesMember_019e4114-b67f-7940-9227-8d7c388a44cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilityAtFairValueAmountRollForward_019e4114-b67f-75fd-8acd-ba6f87d5b0dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilityAtFairValueAmountRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingLiabilitiesAtFairValueLineItems_019e4114-b67f-7db0-a121-f5061109f7c3" xlink:to="loc_us-gaap_ServicingLiabilityAtFairValueAmountRollForward_019e4114-b67f-75fd-8acd-ba6f87d5b0dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-7157-b36a-8a24692e648f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilityAtFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingLiabilityAtFairValueAmountRollForward_019e4114-b67f-75fd-8acd-ba6f87d5b0dd" xlink:to="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-7157-b36a-8a24692e648f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilityAtFairValueAdditions_019e4114-b67f-7c6d-8597-ac4db0ae3e55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilityAtFairValueAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingLiabilityAtFairValueAmountRollForward_019e4114-b67f-75fd-8acd-ba6f87d5b0dd" xlink:to="loc_us-gaap_ServicingLiabilityAtFairValueAdditions_019e4114-b67f-7c6d-8597-ac4db0ae3e55" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ServicingLiabilityAtFairValueRepaymentsAndOtherChangesAtFairValueGainLossIncludedInEarnings_019e4114-b67f-740d-82e2-d44619d85f64" xlink:href="upst-20260630.xsd#upst_ServicingLiabilityAtFairValueRepaymentsAndOtherChangesAtFairValueGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServicingLiabilityAtFairValueAmountRollForward_019e4114-b67f-75fd-8acd-ba6f87d5b0dd" xlink:to="loc_upst_ServicingLiabilityAtFairValueRepaymentsAndOtherChangesAtFairValueGainLossIncludedInEarnings_019e4114-b67f-740d-82e2-d44619d85f64" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServicingLiabilityAtFairValueAmount_019e4114-b67f-7d0d-964f-3754af1c83a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServicingLiabilityAtFairValueAmount"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/GoodwillandIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#GoodwillandIntangibleAssetsScheduleofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/GoodwillandIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e4114-b67f-7508-bc46-cbb439e68f9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e4114-b67f-7591-945d-8672f4643e5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e4114-b67f-7508-bc46-cbb439e68f9d" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e4114-b67f-7591-945d-8672f4643e5d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e4114-b67f-78d2-acdf-6344f5eef2c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e4114-b67f-7591-945d-8672f4643e5d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e4114-b67f-78d2-acdf-6344f5eef2c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e4114-b67f-78d2-acdf-6344f5eef2c1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e4114-b67f-78d2-acdf-6344f5eef2c1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e4114-b67f-78d2-acdf-6344f5eef2c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e4114-b67f-795a-83e8-e02dea53375e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e4114-b67f-78d2-acdf-6344f5eef2c1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e4114-b67f-795a-83e8-e02dea53375e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019e4114-b67f-76b3-a06d-1f892a7ea248" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e4114-b67f-795a-83e8-e02dea53375e" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019e4114-b67f-76b3-a06d-1f892a7ea248" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e4114-b67f-7358-8415-66e9f27af654" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e4114-b67f-7508-bc46-cbb439e68f9d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e4114-b67f-7358-8415-66e9f27af654" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e4114-b67f-737c-b65a-cdbd99925b17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e4114-b67f-7508-bc46-cbb439e68f9d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e4114-b67f-737c-b65a-cdbd99925b17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e4114-b67f-75b0-9df5-d6f75a08bea1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e4114-b67f-7508-bc46-cbb439e68f9d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e4114-b67f-75b0-9df5-d6f75a08bea1" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4114-b67f-7c40-ab62-06f08b905c87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e4114-b67f-72c5-acf3-3b70d684ff07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4114-b67f-7c40-ab62-06f08b905c87" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e4114-b67f-72c5-acf3-3b70d684ff07" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4114-b67f-7839-99e6-ead85b5a540e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e4114-b67f-72c5-acf3-3b70d684ff07" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4114-b67f-7839-99e6-ead85b5a540e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-7839-99e6-ead85b5a540e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4114-b67f-7839-99e6-ead85b5a540e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-7839-99e6-ead85b5a540e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-735c-9f68-8858502a19d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4114-b67f-7839-99e6-ead85b5a540e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-735c-9f68-8858502a19d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareInternallyDevelopedAndPurchasedMember_019f5d14-abc7-7d48-875f-76115906ea5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareInternallyDevelopedAndPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-735c-9f68-8858502a19d7" xlink:to="loc_us-gaap_SoftwareInternallyDevelopedAndPurchasedMember_019f5d14-abc7-7d48-875f-76115906ea5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019e4114-b67f-7f3e-9f31-9997c5a20d20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-735c-9f68-8858502a19d7" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019e4114-b67f-7f3e-9f31-9997c5a20d20" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_019e4114-b67f-7987-b4a6-e6357729b5be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-735c-9f68-8858502a19d7" xlink:to="loc_us-gaap_ComputerEquipmentMember_019e4114-b67f-7987-b4a6-e6357729b5be" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019e4114-b67f-7aaa-a92d-e2171484942a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4114-b67f-735c-9f68-8858502a19d7" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019e4114-b67f-7aaa-a92d-e2171484942a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019e4114-b67f-7467-bd5e-17dc4d890af9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4114-b67f-7c40-ab62-06f08b905c87" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019e4114-b67f-7467-bd5e-17dc4d890af9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e4114-b67f-7632-b7b6-46f271865a9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4114-b67f-7c40-ab62-06f08b905c87" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e4114-b67f-7632-b7b6-46f271865a9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019e4114-b67f-7294-b740-9125bebb86c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4114-b67f-7c40-ab62-06f08b905c87" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019e4114-b67f-7294-b740-9125bebb86c6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BalanceSheetComponentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019ed6a0-ad38-719d-83a6-855bac559298" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019ed6a0-ad38-7e63-a28b-ece35144e8b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019ed6a0-ad38-719d-83a6-855bac559298" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019ed6a0-ad38-7e63-a28b-ece35144e8b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ed6a0-ad38-7a8e-8dbe-e7ab2ab28ff3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019ed6a0-ad38-7e63-a28b-ece35144e8b5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ed6a0-ad38-7a8e-8dbe-e7ab2ab28ff3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ed6a0-ad38-7a8e-8dbe-e7ab2ab28ff3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ed6a0-ad38-7a8e-8dbe-e7ab2ab28ff3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ed6a0-ad38-7a8e-8dbe-e7ab2ab28ff3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ed6a0-ad38-743c-a776-b5cbdf8a618d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ed6a0-ad38-7a8e-8dbe-e7ab2ab28ff3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ed6a0-ad38-743c-a776-b5cbdf8a618d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareInternallyDevelopedAndPurchasedMember_019f5d13-fe0e-7ca8-915e-d44e53ffac7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareInternallyDevelopedAndPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ed6a0-ad38-743c-a776-b5cbdf8a618d" xlink:to="loc_us-gaap_SoftwareInternallyDevelopedAndPurchasedMember_019f5d13-fe0e-7ca8-915e-d44e53ffac7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_019e4114-b67f-7d6f-b08a-ab58e0a803d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019ed6a0-ad38-719d-83a6-855bac559298" xlink:to="loc_us-gaap_Depreciation_019e4114-b67f-7d6f-b08a-ab58e0a803d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019ed6a0-ad38-7c53-8843-74bc1ee71a83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019ed6a0-ad38-719d-83a6-855bac559298" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019ed6a0-ad38-7c53-8843-74bc1ee71a83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TangibleAssetImpairmentCharges_019e4114-b67f-7ab6-b130-982a34befd6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TangibleAssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019ed6a0-ad38-719d-83a6-855bac559298" xlink:to="loc_us-gaap_TangibleAssetImpairmentCharges_019e4114-b67f-7ab6-b130-982a34befd6e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BorrowingsScheduleofAggregatePrincipalOutstandingofallLoansDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BorrowingsScheduleofAggregatePrincipalOutstandingofallLoansDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BorrowingsScheduleofAggregatePrincipalOutstandingofallLoansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7d37-b4ba-7c407d90e32f" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-7c0a-9038-b05a88e9fb57" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7d37-b4ba-7c407d90e32f" xlink:to="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-7c0a-9038-b05a88e9fb57" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-7ab9-8d42-305cd93f8c77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-7c0a-9038-b05a88e9fb57" xlink:to="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-7ab9-8d42-305cd93f8c77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-7ab9-8d42-305cd93f8c77_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-7ab9-8d42-305cd93f8c77" xlink:to="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-7ab9-8d42-305cd93f8c77_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-700b-862c-7a354e4a1b50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-7ab9-8d42-305cd93f8c77" xlink:to="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-700b-862c-7a354e4a1b50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e4114-b67f-7f61-bfe9-e889a5290bae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-700b-862c-7a354e4a1b50" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e4114-b67f-7f61-bfe9-e889a5290bae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7e2d-a687-c9f2ed8a1013" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-7c0a-9038-b05a88e9fb57" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7e2d-a687-c9f2ed8a1013" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7e2d-a687-c9f2ed8a1013_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7e2d-a687-c9f2ed8a1013" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7e2d-a687-c9f2ed8a1013_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7299-ae80-3dc15133c0f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7e2d-a687-c9f2ed8a1013" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7299-ae80-3dc15133c0f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_WarehouseCreditFacilityMember_019e4114-b67f-7152-8e63-83347018401f" xlink:href="upst-20260630.xsd#upst_WarehouseCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7299-ae80-3dc15133c0f9" xlink:to="loc_upst_WarehouseCreditFacilityMember_019e4114-b67f-7152-8e63-83347018401f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RiskRetentionFinancingFacilityMember_019e4114-b67f-7ee9-b82e-feb26eb325e6" xlink:href="upst-20260630.xsd#upst_RiskRetentionFinancingFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7299-ae80-3dc15133c0f9" xlink:to="loc_upst_RiskRetentionFinancingFacilityMember_019e4114-b67f-7ee9-b82e-feb26eb325e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7e94-a713-8e95d9ed67a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-7c0a-9038-b05a88e9fb57" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7e94-a713-8e95d9ed67a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7e94-a713-8e95d9ed67a0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7e94-a713-8e95d9ed67a0" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7e94-a713-8e95d9ed67a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7432-b375-56da9d4df8d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7e94-a713-8e95d9ed67a0" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7432-b375-56da9d4df8d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e4114-b67f-738e-b72f-d9509c062bb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7432-b375-56da9d4df8d6" xlink:to="loc_us-gaap_SecuredDebtMember_019e4114-b67f-738e-b72f-d9509c062bb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e4114-b67f-743e-872f-d6207d24d613" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7432-b375-56da9d4df8d6" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e4114-b67f-743e-872f-d6207d24d613" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LongTermAndShortTermDebtGross_019e4114-b67f-77e6-b792-e458d35bd268" xlink:href="upst-20260630.xsd#upst_LongTermAndShortTermDebtGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7d37-b4ba-7c407d90e32f" xlink:to="loc_upst_LongTermAndShortTermDebtGross_019e4114-b67f-77e6-b792-e458d35bd268" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e4114-b67f-757b-83ff-dce5a8efd56e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7d37-b4ba-7c407d90e32f" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e4114-b67f-757b-83ff-dce5a8efd56e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019e4114-b67f-78d7-a987-f8f4078c15e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7d37-b4ba-7c407d90e32f" xlink:to="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019e4114-b67f-78d7-a987-f8f4078c15e9" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BorrowingsScheduleofWarehouseCreditFacilitiesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BorrowingsScheduleofWarehouseCreditFacilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BorrowingsScheduleofWarehouseCreditFacilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_019e4114-b67f-72c0-ba11-6b41fbbbbde5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_019e4114-b67f-72c0-ba11-6b41fbbbbde5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-765b-95b7-3db8b2d6349c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e4114-b67f-72c0-ba11-6b41fbbbbde5" xlink:to="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-765b-95b7-3db8b2d6349c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-765b-95b7-3db8b2d6349c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-765b-95b7-3db8b2d6349c" xlink:to="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-765b-95b7-3db8b2d6349c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-7fad-838c-3dab49352ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e4114-b67f-765b-95b7-3db8b2d6349c" xlink:to="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-7fad-838c-3dab49352ce5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e4114-b67f-7a24-ba7b-a42dfd80ba83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e4114-b67f-7fad-838c-3dab49352ce5" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e4114-b67f-7a24-ba7b-a42dfd80ba83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-79c7-8d41-cedc3289afd2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e4114-b67f-72c0-ba11-6b41fbbbbde5" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-79c7-8d41-cedc3289afd2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-79c7-8d41-cedc3289afd2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-79c7-8d41-cedc3289afd2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-79c7-8d41-cedc3289afd2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-79c7-8d41-cedc3289afd2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_UAWTWarehouseCreditFacilityIIMember_019e4114-b67f-7bf8-984a-ece9c15a9e00" xlink:href="upst-20260630.xsd#upst_UAWTWarehouseCreditFacilityIIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:to="loc_upst_UAWTWarehouseCreditFacilityIIMember_019e4114-b67f-7bf8-984a-ece9c15a9e00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ULTWarehouseCreditFacilityMember_019e4114-b67f-7f8c-a3c7-0059a618b1bd" xlink:href="upst-20260630.xsd#upst_ULTWarehouseCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:to="loc_upst_ULTWarehouseCreditFacilityMember_019e4114-b67f-7f8c-a3c7-0059a618b1bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_UpstartSmallDollarLoanTrustMember_019e4114-b67f-7066-901f-0ef8b28417f2" xlink:href="upst-20260630.xsd#upst_UpstartSmallDollarLoanTrustMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:to="loc_upst_UpstartSmallDollarLoanTrustMember_019e4114-b67f-7066-901f-0ef8b28417f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_WarehouseCreditFacilityMember_019e4114-b67f-7f79-a054-edb09c186f95" xlink:href="upst-20260630.xsd#upst_WarehouseCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:to="loc_upst_WarehouseCreditFacilityMember_019e4114-b67f-7f79-a054-edb09c186f95" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ULTWarehouseCreditFacilityUnCommittedMember_019e4114-b67f-7105-901d-96e8d28f66f8" xlink:href="upst-20260630.xsd#upst_ULTWarehouseCreditFacilityUnCommittedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7dc5-bbae-1916507e482c" xlink:to="loc_upst_ULTWarehouseCreditFacilityUnCommittedMember_019e4114-b67f-7105-901d-96e8d28f66f8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019e4114-b67f-7006-a553-3f7e2dd94723" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e4114-b67f-72c0-ba11-6b41fbbbbde5" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019e4114-b67f-7006-a553-3f7e2dd94723" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b67f-7006-a553-3f7e2dd94723_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b67f-7006-a553-3f7e2dd94723" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b67f-7006-a553-3f7e2dd94723_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019e4114-b67f-793d-9c8e-d3a8094653b7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019e4114-b67f-7006-a553-3f7e2dd94723" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019e4114-b67f-793d-9c8e-d3a8094653b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b67f-75e1-83db-1ae542ffe309" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019e4114-b67f-793d-9c8e-d3a8094653b7" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019e4114-b67f-75e1-83db-1ae542ffe309" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e4114-b67f-71c9-8f03-b3aa3534f095" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e4114-b67f-72c0-ba11-6b41fbbbbde5" xlink:to="loc_srt_RangeAxis_019e4114-b67f-71c9-8f03-b3aa3534f095" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-71c9-8f03-b3aa3534f095_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e4114-b67f-71c9-8f03-b3aa3534f095" xlink:to="loc_srt_RangeMember_019e4114-b67f-71c9-8f03-b3aa3534f095_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-70dc-a872-a59c2a4d7441" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e4114-b67f-71c9-8f03-b3aa3534f095" xlink:to="loc_srt_RangeMember_019e4114-b67f-70dc-a872-a59c2a4d7441" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e4114-b67f-7ea9-a5f5-c2befeb1de8e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-70dc-a872-a59c2a4d7441" xlink:to="loc_srt_MinimumMember_019e4114-b67f-7ea9-a5f5-c2befeb1de8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e4114-b67f-7cc4-a94d-8740cf232ea5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-70dc-a872-a59c2a4d7441" xlink:to="loc_srt_MaximumMember_019e4114-b67f-7cc4-a94d-8740cf232ea5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e4114-b67f-7730-bf23-0dc7122422f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e4114-b67f-7730-bf23-0dc7122422f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e4114-b67f-7770-9f98-0744239577af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e4114-b67f-7770-9f98-0744239577af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCollateralAmount_019e4114-b67f-7887-bced-70198d5b91fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCollateralAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_us-gaap_DebtInstrumentCollateralAmount_019e4114-b67f-7887-bced-70198d5b91fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LongTermAndShortTermDebtGross_019e4114-b67f-77bb-9924-0f5a8d5ddd27" xlink:href="upst-20260630.xsd#upst_LongTermAndShortTermDebtGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_upst_LongTermAndShortTermDebtGross_019e4114-b67f-77bb-9924-0f5a8d5ddd27" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e4114-b67f-70f1-a933-5261981a8ead" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e4114-b67f-70f1-a933-5261981a8ead" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LineOfCreditFacilityUnusedCapacityCommitmentFeeThresholdPercentageOfBorrowingBase_019e4114-b67f-73f9-8dd3-9dd30a811efc" xlink:href="upst-20260630.xsd#upst_LineOfCreditFacilityUnusedCapacityCommitmentFeeThresholdPercentageOfBorrowingBase"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e4114-b67f-7e7f-a21b-4a33613476ec" xlink:to="loc_upst_LineOfCreditFacilityUnusedCapacityCommitmentFeeThresholdPercentageOfBorrowingBase_019e4114-b67f-73f9-8dd3-9dd30a811efc" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BorrowingsNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BorrowingsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BorrowingsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-7b11-9345-50797b96e567_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:to="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-7b11-9345-50797b96e567_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-76ff-9045-366158f02845" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:to="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-76ff-9045-366158f02845" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019f6140-3cc6-7c71-abd2-3566833f76e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-76ff-9045-366158f02845" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019f6140-3cc6-7c71-abd2-3566833f76e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-776e-a781-f6d5b5f611f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-776e-a781-f6d5b5f611f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ULTWarehouseCreditFacilityMember_019f6140-3cc6-7a95-8949-bf0dd05eec04" xlink:href="upst-20260630.xsd#upst_ULTWarehouseCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_ULTWarehouseCreditFacilityMember_019f6140-3cc6-7a95-8949-bf0dd05eec04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RiskRetentionFinancingFacilityMember_019f6140-97ba-7df0-9645-1be1fe7664e5" xlink:href="upst-20260630.xsd#upst_RiskRetentionFinancingFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_RiskRetentionFinancingFacilityMember_019f6140-97ba-7df0-9645-1be1fe7664e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentySixNotesMember_019f6140-d221-7c77-9b93-a275487a383b" xlink:href="upst-20260630.xsd#upst_TwentyTwentySixNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyTwentySixNotesMember_019f6140-d221-7c77-9b93-a275487a383b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentyNineNotesMember_019f6140-d221-7310-a041-c1bfcd1b4a67" xlink:href="upst-20260630.xsd#upst_TwentyTwentyNineNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyTwentyNineNotesMember_019f6140-d221-7310-a041-c1bfcd1b4a67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyNotesMember_019f6140-d221-7467-b203-b7d9652ea879" xlink:href="upst-20260630.xsd#upst_TwentyThirtyNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyThirtyNotesMember_019f6140-d221-7467-b203-b7d9652ea879" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyTwoNotesMember_019f6140-d221-7a54-b47e-c713349a6592" xlink:href="upst-20260630.xsd#upst_TwentyThirtyTwoNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyThirtyTwoNotesMember_019f6140-d221-7a54-b47e-c713349a6592" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-79a0-ba8d-1be1b42e5fab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-79a0-ba8d-1be1b42e5fab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019f6140-3cc6-733a-b746-389115e9e42b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:to="loc_us-gaap_SecuredDebtMember_019f6140-3cc6-733a-b746-389115e9e42b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019f6140-d221-7f3e-a53c-09e3d8132bc1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:to="loc_us-gaap_ConvertibleDebtMember_019f6140-d221-7f3e-a53c-09e3d8132bc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019f6140-97ba-792f-83a0-abc5a3e9ad13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfFinancialPositionLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019f6140-97ba-792f-83a0-abc5a3e9ad13" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-791e-8771-637dc48e2f97" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-791e-8771-637dc48e2f97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019f6140-97ba-7064-b0fd-f0a5ed7dbaa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-791e-8771-637dc48e2f97" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019f6140-97ba-7064-b0fd-f0a5ed7dbaa2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-749f-bfe8-437b422fc99c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-749f-bfe8-437b422fc99c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-79d1-912d-702489a1afac" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-79d1-912d-702489a1afac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConsolidatedSecuritizationMember_019f6140-97ba-7549-b93a-28028ae305fe" xlink:href="upst-20260630.xsd#upst_ConsolidatedSecuritizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-79d1-912d-702489a1afac" xlink:to="loc_upst_ConsolidatedSecuritizationMember_019f6140-97ba-7549-b93a-28028ae305fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConversionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-72df-b6d0-ec3aff7c2ecb_default" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConversionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:to="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-72df-b6d0-ec3aff7c2ecb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConversionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:to="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConversionPeriodOneMember_019f6141-75db-7281-8292-1ba5cfc4b893" xlink:href="upst-20260630.xsd#upst_ConversionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:to="loc_upst_ConversionPeriodOneMember_019f6141-75db-7281-8292-1ba5cfc4b893" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConversionPeriodTwoMember_019f6141-75db-7289-992c-65c17f3c26c9" xlink:href="upst-20260630.xsd#upst_ConversionPeriodTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:to="loc_upst_ConversionPeriodTwoMember_019f6141-75db-7289-992c-65c17f3c26c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019f6140-3cc6-700c-91bd-0c9d8e188b74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019f6140-3cc6-700c-91bd-0c9d8e188b74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LongTermAndShortTermDebtGross_019f6140-3cc6-7cea-8d00-e901fe529349" xlink:href="upst-20260630.xsd#upst_LongTermAndShortTermDebtGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_LongTermAndShortTermDebtGross_019f6140-3cc6-7cea-8d00-e901fe529349" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019f6140-97ba-773b-b944-5eb0691c873d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019f6140-97ba-773b-b944-5eb0691c873d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuritiesLoanedFairValueOfCollateral_019f6140-97ba-72bd-9e35-e03e9371281c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuritiesLoanedFairValueOfCollateral"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_SecuritiesLoanedFairValueOfCollateral_019f6140-97ba-72bd-9e35-e03e9371281c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtWeightedAverageInterestRate_019f6140-97ba-7b89-bed2-a7fb2f29d869" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtWeightedAverageInterestRate_019f6140-97ba-7b89-bed2-a7fb2f29d869" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f6140-d221-7f20-8d1e-baddf02a42b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f6140-d221-7f20-8d1e-baddf02a42b9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_019f6140-d221-72e4-803b-23af718fbac3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_019f6140-d221-72e4-803b-23af718fbac3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_019f6140-d221-7fc4-b163-1e35c3330035" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRepurchasedFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_019f6140-d221-7fc4-b163-1e35c3330035" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentAdditionalRepurchasedFaceAmount_019f6140-d222-7364-b2d0-4e024e098a98" xlink:href="upst-20260630.xsd#upst_DebtInstrumentAdditionalRepurchasedFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_DebtInstrumentAdditionalRepurchasedFaceAmount_019f6140-d222-7364-b2d0-4e024e098a98" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019f6140-d222-78e1-b0da-606bf6cb7944" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019f6140-d222-78e1-b0da-606bf6cb7944" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019f6140-d222-7ea0-96ad-27ba82e39329" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019f6140-d222-7ea0-96ad-27ba82e39329" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019f6140-d222-72ec-ab6a-bb6d9572a8d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019f6140-d222-72ec-ab6a-bb6d9572a8d4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019f6140-d222-777c-8887-f13d0d4251b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019f6140-d222-777c-8887-f13d0d4251b6" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019f6141-2748-7b53-a591-1146718e946d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019f6141-2748-7b53-a591-1146718e946d" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConvertibleMaximumPercentageOfProductOfLastReportedSalePriceAndConversionRate_019f6141-75db-738d-a344-94382dd259dc" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConvertibleMaximumPercentageOfProductOfLastReportedSalePriceAndConversionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_DebtInstrumentConvertibleMaximumPercentageOfProductOfLastReportedSalePriceAndConversionRate_019f6141-75db-738d-a344-94382dd259dc" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019f6141-a76e-7016-9379-3e29e3061740" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019f6141-a76e-7016-9379-3e29e3061740" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019f6141-a76e-710a-b599-7b4f0ed742f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019f6141-a76e-710a-b599-7b4f0ed742f4" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019fb48c-5a5b-72bb-96c1-3bc89b015bc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019fb48c-5a5b-72bb-96c1-3bc89b015bc0" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BorrowingsNarrativeDetails_1" xlink:type="simple" xlink:href="upst-20260630.xsd#BorrowingsNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BorrowingsNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-7b11-9345-50797b96e567_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:to="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-7b11-9345-50797b96e567_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-76ff-9045-366158f02845" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019f6140-3cc6-7b11-9345-50797b96e567" xlink:to="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-76ff-9045-366158f02845" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019f6140-3cc6-7c71-abd2-3566833f76e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019f6140-3cc6-76ff-9045-366158f02845" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019f6140-3cc6-7c71-abd2-3566833f76e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-776e-a781-f6d5b5f611f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-776e-a781-f6d5b5f611f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f6140-3cc6-776e-a781-f6d5b5f611f1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ULTWarehouseCreditFacilityMember_019f6140-3cc6-7a95-8949-bf0dd05eec04" xlink:href="upst-20260630.xsd#upst_ULTWarehouseCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_ULTWarehouseCreditFacilityMember_019f6140-3cc6-7a95-8949-bf0dd05eec04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RiskRetentionFinancingFacilityMember_019f6140-97ba-7df0-9645-1be1fe7664e5" xlink:href="upst-20260630.xsd#upst_RiskRetentionFinancingFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_RiskRetentionFinancingFacilityMember_019f6140-97ba-7df0-9645-1be1fe7664e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentySixNotesMember_019f6140-d221-7c77-9b93-a275487a383b" xlink:href="upst-20260630.xsd#upst_TwentyTwentySixNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyTwentySixNotesMember_019f6140-d221-7c77-9b93-a275487a383b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentyNineNotesMember_019f6140-d221-7310-a041-c1bfcd1b4a67" xlink:href="upst-20260630.xsd#upst_TwentyTwentyNineNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyTwentyNineNotesMember_019f6140-d221-7310-a041-c1bfcd1b4a67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyNotesMember_019f6140-d221-7467-b203-b7d9652ea879" xlink:href="upst-20260630.xsd#upst_TwentyThirtyNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyThirtyNotesMember_019f6140-d221-7467-b203-b7d9652ea879" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyTwoNotesMember_019f6140-d221-7a54-b47e-c713349a6592" xlink:href="upst-20260630.xsd#upst_TwentyThirtyTwoNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f6140-3cc6-7035-9eed-75acd12185f2" xlink:to="loc_upst_TwentyThirtyTwoNotesMember_019f6140-d221-7a54-b47e-c713349a6592" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-79a0-ba8d-1be1b42e5fab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-79a0-ba8d-1be1b42e5fab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f6140-3cc6-79a0-ba8d-1be1b42e5fab" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019f6140-3cc6-733a-b746-389115e9e42b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:to="loc_us-gaap_SecuredDebtMember_019f6140-3cc6-733a-b746-389115e9e42b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019f6140-d221-7f3e-a53c-09e3d8132bc1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f6140-3cc6-7a4e-ab25-010c7bd92a53" xlink:to="loc_us-gaap_ConvertibleDebtMember_019f6140-d221-7f3e-a53c-09e3d8132bc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-791e-8771-637dc48e2f97" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f6140-97ba-7410-ad8f-2a9cbe8dcdc3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-791e-8771-637dc48e2f97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019f6140-97ba-7064-b0fd-f0a5ed7dbaa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019f6140-97ba-791e-8771-637dc48e2f97" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019f6140-97ba-7064-b0fd-f0a5ed7dbaa2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-749f-bfe8-437b422fc99c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-749f-bfe8-437b422fc99c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-79d1-912d-702489a1afac" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019f6140-97ba-749f-bfe8-437b422fc99c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-79d1-912d-702489a1afac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConsolidatedSecuritizationMember_019f6140-97ba-7549-b93a-28028ae305fe" xlink:href="upst-20260630.xsd#upst_ConsolidatedSecuritizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f6140-97ba-79d1-912d-702489a1afac" xlink:to="loc_upst_ConsolidatedSecuritizationMember_019f6140-97ba-7549-b93a-28028ae305fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConversionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019f6140-3cc6-7fdb-937d-8c64952269b2" xlink:to="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-72df-b6d0-ec3aff7c2ecb_default" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConversionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:to="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-72df-b6d0-ec3aff7c2ecb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConversionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_upst_DebtInstrumentConversionAxis_019f6141-75db-72df-b6d0-ec3aff7c2ecb" xlink:to="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConversionPeriodOneMember_019f6141-75db-7281-8292-1ba5cfc4b893" xlink:href="upst-20260630.xsd#upst_ConversionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:to="loc_upst_ConversionPeriodOneMember_019f6141-75db-7281-8292-1ba5cfc4b893" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ConversionPeriodTwoMember_019f6141-75db-7289-992c-65c17f3c26c9" xlink:href="upst-20260630.xsd#upst_ConversionPeriodTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_DebtInstrumentConversionDomain_019f6141-75db-7df5-8b61-e6a599e90f4b" xlink:to="loc_upst_ConversionPeriodTwoMember_019f6141-75db-7289-992c-65c17f3c26c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019f6140-3cc6-700c-91bd-0c9d8e188b74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019f6140-3cc6-700c-91bd-0c9d8e188b74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LongTermAndShortTermDebtGross_019f6140-3cc6-7cea-8d00-e901fe529349" xlink:href="upst-20260630.xsd#upst_LongTermAndShortTermDebtGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_LongTermAndShortTermDebtGross_019f6140-3cc6-7cea-8d00-e901fe529349" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019f6140-97ba-773b-b944-5eb0691c873d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019f6140-97ba-773b-b944-5eb0691c873d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuritiesLoanedFairValueOfCollateral_019f6140-97ba-72bd-9e35-e03e9371281c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuritiesLoanedFairValueOfCollateral"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_SecuritiesLoanedFairValueOfCollateral_019f6140-97ba-72bd-9e35-e03e9371281c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtWeightedAverageInterestRate_019f6140-97ba-7b89-bed2-a7fb2f29d869" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtWeightedAverageInterestRate_019f6140-97ba-7b89-bed2-a7fb2f29d869" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f6140-d221-7f20-8d1e-baddf02a42b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f6140-d221-7f20-8d1e-baddf02a42b9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_019f6140-d221-72e4-803b-23af718fbac3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_019f6140-d221-72e4-803b-23af718fbac3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_019f6140-d221-7fc4-b163-1e35c3330035" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRepurchasedFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_019f6140-d221-7fc4-b163-1e35c3330035" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentAdditionalRepurchasedFaceAmount_019f6140-d222-7364-b2d0-4e024e098a98" xlink:href="upst-20260630.xsd#upst_DebtInstrumentAdditionalRepurchasedFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_DebtInstrumentAdditionalRepurchasedFaceAmount_019f6140-d222-7364-b2d0-4e024e098a98" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019f6140-d222-78e1-b0da-606bf6cb7944" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019f6140-d222-78e1-b0da-606bf6cb7944" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019f6140-d222-7ea0-96ad-27ba82e39329" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019f6140-d222-7ea0-96ad-27ba82e39329" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019f6140-d222-72ec-ab6a-bb6d9572a8d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019f6140-d222-72ec-ab6a-bb6d9572a8d4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019f6140-d222-777c-8887-f13d0d4251b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019f6140-d222-777c-8887-f13d0d4251b6" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019f6141-2748-7b53-a591-1146718e946d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019f6141-2748-7b53-a591-1146718e946d" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConvertibleMaximumPercentageOfProductOfLastReportedSalePriceAndConversionRate_019f6141-75db-738d-a344-94382dd259dc" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConvertibleMaximumPercentageOfProductOfLastReportedSalePriceAndConversionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_upst_DebtInstrumentConvertibleMaximumPercentageOfProductOfLastReportedSalePriceAndConversionRate_019f6141-75db-738d-a344-94382dd259dc" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019f6141-a76e-7016-9379-3e29e3061740" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019f6141-a76e-7016-9379-3e29e3061740" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019f6141-a76e-710a-b599-7b4f0ed742f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019f6141-a76e-710a-b599-7b4f0ed742f4" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019fb48c-5a5b-72bb-96c1-3bc89b015bc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019f6140-3cc6-7913-9c2b-eacd2a795e8f" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019fb48c-5a5b-72bb-96c1-3bc89b015bc0" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BorrowingsScheduleofConvertibleSeniorNotesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BorrowingsScheduleofConvertibleSeniorNotesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BorrowingsScheduleofConvertibleSeniorNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e4114-b67f-73f5-885b-1e8427d070b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentTable_019e4114-b67f-73f5-885b-1e8427d070b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7a42-809e-e7e91f6fb944" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e4114-b67f-73f5-885b-1e8427d070b1" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7a42-809e-e7e91f6fb944" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7a42-809e-e7e91f6fb944_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7a42-809e-e7e91f6fb944" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7a42-809e-e7e91f6fb944_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7394-a33d-b9225e80e97d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-7a42-809e-e7e91f6fb944" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7394-a33d-b9225e80e97d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentySixNotesMember_019e4114-b67f-7860-94ff-aa9c7c70644c" xlink:href="upst-20260630.xsd#upst_TwentyTwentySixNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7394-a33d-b9225e80e97d" xlink:to="loc_upst_TwentyTwentySixNotesMember_019e4114-b67f-7860-94ff-aa9c7c70644c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentyNineNotesMember_019e4114-b67f-786a-a52e-33526fcdd331" xlink:href="upst-20260630.xsd#upst_TwentyTwentyNineNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7394-a33d-b9225e80e97d" xlink:to="loc_upst_TwentyTwentyNineNotesMember_019e4114-b67f-786a-a52e-33526fcdd331" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyNotesMember_019e4114-b67f-7719-aeff-c959e81d99b8" xlink:href="upst-20260630.xsd#upst_TwentyThirtyNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7394-a33d-b9225e80e97d" xlink:to="loc_upst_TwentyThirtyNotesMember_019e4114-b67f-7719-aeff-c959e81d99b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyTwoNotesMember_019e4114-b67f-756d-a7bd-fdc8190f3901" xlink:href="upst-20260630.xsd#upst_TwentyThirtyTwoNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-7394-a33d-b9225e80e97d" xlink:to="loc_upst_TwentyThirtyTwoNotesMember_019e4114-b67f-756d-a7bd-fdc8190f3901" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7736-b0e4-10a2d7352172" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e4114-b67f-73f5-885b-1e8427d070b1" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7736-b0e4-10a2d7352172" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7736-b0e4-10a2d7352172_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7736-b0e4-10a2d7352172" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7736-b0e4-10a2d7352172_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7cf0-b496-299d3c77a2e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4114-b67f-7736-b0e4-10a2d7352172" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7cf0-b496-299d3c77a2e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e4114-b67f-7276-9665-2533be667411" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e4114-b67f-7cf0-b496-299d3c77a2e1" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e4114-b67f-7276-9665-2533be667411" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e4114-b67f-7760-b531-31198046829c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e4114-b67f-7760-b531-31198046829c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e4114-b67f-74c5-b294-034ce54f9cee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e4114-b67f-74c5-b294-034ce54f9cee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019e4114-b67f-7a59-a3ff-b98bf9bfd1f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019e4114-b67f-7a59-a3ff-b98bf9bfd1f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e4114-b67f-7b32-a42d-31fa699161c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e4114-b67f-7b32-a42d-31fa699161c7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e4114-b67f-7c36-8830-698767407e22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e4114-b67f-7c36-8830-698767407e22" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e4114-b67f-7564-bfb5-9f082ed3cf01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_LongTermDebt_019e4114-b67f-7564-bfb5-9f082ed3cf01" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_019e4114-b67f-7747-bd25-97cdaa0c6b7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4114-b67f-75eb-8ea7-3edce50a18ef" xlink:to="loc_us-gaap_DebtInstrumentFairValue_019e4114-b67f-7747-bd25-97cdaa0c6b7b" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/BorrowingsScheduleofCappedCallTransactionsDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#BorrowingsScheduleofCappedCallTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/BorrowingsScheduleofCappedCallTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7a52-b687-b7074e685dd4" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-77e9-8ec3-2dd8d01b01a8" xlink:href="upst-20260630.xsd#upst_ScheduleOfBorrowingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7a52-b687-b7074e685dd4" xlink:to="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-77e9-8ec3-2dd8d01b01a8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-71bf-82b6-032c2e9fc667" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_upst_ScheduleOfBorrowingsTable_019e4114-b67f-77e9-8ec3-2dd8d01b01a8" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-71bf-82b6-032c2e9fc667" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-71bf-82b6-032c2e9fc667_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-71bf-82b6-032c2e9fc667" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-71bf-82b6-032c2e9fc667_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-78de-9509-a1c2131813d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4114-b67f-71bf-82b6-032c2e9fc667" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-78de-9509-a1c2131813d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentySixNotesMember_019e4114-b67f-7791-96fa-66ea4e4c2930" xlink:href="upst-20260630.xsd#upst_TwentyTwentySixNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-78de-9509-a1c2131813d6" xlink:to="loc_upst_TwentyTwentySixNotesMember_019e4114-b67f-7791-96fa-66ea4e4c2930" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyTwentyNineNotesMember_019e4114-b67f-79ff-b9e6-1d4224fd4faf" xlink:href="upst-20260630.xsd#upst_TwentyTwentyNineNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-78de-9509-a1c2131813d6" xlink:to="loc_upst_TwentyTwentyNineNotesMember_019e4114-b67f-79ff-b9e6-1d4224fd4faf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_TwentyThirtyTwoNotesMember_019e4114-b67f-7f56-9c84-ee2344496868" xlink:href="upst-20260630.xsd#upst_TwentyThirtyTwoNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4114-b67f-78de-9509-a1c2131813d6" xlink:to="loc_upst_TwentyThirtyTwoNotesMember_019e4114-b67f-7f56-9c84-ee2344496868" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConvertibleCappedCallsInitialStrikePrice_019e4114-b67f-7b95-90c3-d7998559318d" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConvertibleCappedCallsInitialStrikePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7a52-b687-b7074e685dd4" xlink:to="loc_upst_DebtInstrumentConvertibleCappedCallsInitialStrikePrice_019e4114-b67f-7b95-90c3-d7998559318d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConvertibleCappedCallsInitialCapPrice_019e4114-b67f-7385-873a-9a315e2adbca" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConvertibleCappedCallsInitialCapPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7a52-b687-b7074e685dd4" xlink:to="loc_upst_DebtInstrumentConvertibleCappedCallsInitialCapPrice_019e4114-b67f-7385-873a-9a315e2adbca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_DebtInstrumentConvertibleCappedCallsNumberOfSharesOfCommonStockCovered_019e4114-b67f-71a0-b286-e4e22cff5f8c" xlink:href="upst-20260630.xsd#upst_DebtInstrumentConvertibleCappedCallsNumberOfSharesOfCommonStockCovered"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_upst_ScheduleOfBorrowingsLineItems_019e4114-b67f-7a52-b687-b7074e685dd4" xlink:to="loc_upst_DebtInstrumentConvertibleCappedCallsNumberOfSharesOfCommonStockCovered_019e4114-b67f-71a0-b286-e4e22cff5f8c" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/StockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#StockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/StockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7765-aaad-4a0bb0b9bbfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7765-aaad-4a0bb0b9bbfd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019e4114-b67f-7953-871a-79ab4afbbfaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7765-aaad-4a0bb0b9bbfd" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019e4114-b67f-7953-871a-79ab4afbbfaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e4114-b67f-7953-871a-79ab4afbbfaa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e4114-b67f-7953-871a-79ab4afbbfaa" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e4114-b67f-7953-871a-79ab4afbbfaa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e4114-b67f-729e-81c6-bb1e04b03417" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e4114-b67f-7953-871a-79ab4afbbfaa" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e4114-b67f-729e-81c6-bb1e04b03417" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AtTheMarketOfferingProgramMember_019e4114-b67f-7064-bb54-98aa6484a6d8" xlink:href="upst-20260630.xsd#upst_AtTheMarketOfferingProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e4114-b67f-729e-81c6-bb1e04b03417" xlink:to="loc_upst_AtTheMarketOfferingProgramMember_019e4114-b67f-7064-bb54-98aa6484a6d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e4114-b67f-754a-a7cb-7645264cb026" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7765-aaad-4a0bb0b9bbfd" xlink:to="loc_srt_RangeAxis_019e4114-b67f-754a-a7cb-7645264cb026" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-754a-a7cb-7645264cb026_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e4114-b67f-754a-a7cb-7645264cb026" xlink:to="loc_srt_RangeMember_019e4114-b67f-754a-a7cb-7645264cb026_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-7c18-84f2-79f54ea9a60f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e4114-b67f-754a-a7cb-7645264cb026" xlink:to="loc_srt_RangeMember_019e4114-b67f-7c18-84f2-79f54ea9a60f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e4114-b67f-77c3-b80c-73b5b6d59449" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7c18-84f2-79f54ea9a60f" xlink:to="loc_srt_MinimumMember_019e4114-b67f-77c3-b80c-73b5b6d59449" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e4114-b67f-7cde-ac20-aede75b4acae" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7c18-84f2-79f54ea9a60f" xlink:to="loc_srt_MaximumMember_019e4114-b67f-7cde-ac20-aede75b4acae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e4114-b67f-774d-accc-ca01d345bb8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7765-aaad-4a0bb0b9bbfd" xlink:to="loc_us-gaap_AwardTypeAxis_019e4114-b67f-774d-accc-ca01d345bb8f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-774d-accc-ca01d345bb8f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-774d-accc-ca01d345bb8f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-774d-accc-ca01d345bb8f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-71a6-99a7-b043c7a4659f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-774d-accc-ca01d345bb8f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-71a6-99a7-b043c7a4659f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-7cb0-8fc2-2b1858c5da09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-71a6-99a7-b043c7a4659f" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-7cb0-8fc2-2b1858c5da09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-7859-a7ec-233e3d6901bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-71a6-99a7-b043c7a4659f" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-7859-a7ec-233e3d6901bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-77ae-97d2-1b4b79002e4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-71a6-99a7-b043c7a4659f" xlink:to="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-77ae-97d2-1b4b79002e4e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7858-9e97-df1060fde7cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-71a6-99a7-b043c7a4659f" xlink:to="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7858-9e97-df1060fde7cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b67f-70cf-83b8-f6750520d134" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7765-aaad-4a0bb0b9bbfd" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b67f-70cf-83b8-f6750520d134" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e4114-b67f-70cf-83b8-f6750520d134_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b67f-70cf-83b8-f6750520d134" xlink:to="loc_us-gaap_EquityComponentDomain_019e4114-b67f-70cf-83b8-f6750520d134_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e4114-b67f-73f7-9082-be116b6679da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e4114-b67f-70cf-83b8-f6750520d134" xlink:to="loc_us-gaap_EquityComponentDomain_019e4114-b67f-73f7-9082-be116b6679da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e4114-b67f-736e-94b2-a775b14cdf7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4114-b67f-73f7-9082-be116b6679da" xlink:to="loc_us-gaap_CommonStockMember_019e4114-b67f-736e-94b2-a775b14cdf7d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019e4114-b67f-7777-8b46-d725494d99f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019e4114-b67f-7777-8b46-d725494d99f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e4114-b67f-7ad0-bb80-56460906e3bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e4114-b67f-7ad0-bb80-56460906e3bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019e4114-b67f-77a1-a61c-2be99b436270" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019e4114-b67f-77a1-a61c-2be99b436270" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e4114-b67f-7345-92a3-038aa950af9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e4114-b67f-7345-92a3-038aa950af9c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_StockRepurchasedAndRetiredDuringPeriodValueNetOfExciseTax_019e4114-b67f-7a65-b02c-d74d08a18c82" xlink:href="upst-20260630.xsd#upst_StockRepurchasedAndRetiredDuringPeriodValueNetOfExciseTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_upst_StockRepurchasedAndRetiredDuringPeriodValueNetOfExciseTax_019e4114-b67f-7a65-b02c-d74d08a18c82" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019e4114-b67f-7f52-b294-aa9de1ff9832" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019e4114-b67f-7f52-b294-aa9de1ff9832" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019e4114-b67f-7e92-80ac-34b510d762f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019e4114-b67f-7e92-80ac-34b510d762f5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SaleOfStockMaximumValueOfSharesToBeSold_019e4114-b67f-77c2-b1ad-acc05613460d" xlink:href="upst-20260630.xsd#upst_SaleOfStockMaximumValueOfSharesToBeSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_upst_SaleOfStockMaximumValueOfSharesToBeSold_019e4114-b67f-77c2-b1ad-acc05613460d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_SaleOfStockCommissionsPercentage_019e4114-b67f-7baf-bb39-95c43ac2e354" xlink:href="upst-20260630.xsd#upst_SaleOfStockCommissionsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_upst_SaleOfStockCommissionsPercentage_019e4114-b67f-7baf-bb39-95c43ac2e354" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019e4114-b67f-7394-823d-5c85b11905ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019e4114-b67f-7394-823d-5c85b11905ec" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019e4114-b67f-7cfb-94ad-f1de0fee6162" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019e4114-b67f-7cfb-94ad-f1de0fee6162" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e4114-b67f-78a0-becf-bb965185dc2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e4114-b67f-78a0-becf-bb965185dc2a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019e4114-b67f-7528-8dfe-129479280403" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019e4114-b67f-7528-8dfe-129479280403" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019e4114-b67f-740a-838a-7a7ef3bb9d11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019e4114-b67f-740a-838a-7a7ef3bb9d11" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e4114-b67f-7773-a878-cf5eb74abc84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e4114-b67f-7773-a878-cf5eb74abc84" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f60a1-111d-7aab-abd3-deee48d60789" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f60a1-111d-7aab-abd3-deee48d60789" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e4114-b67f-700f-99fb-65169b53f6cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e4114-b67f-700f-99fb-65169b53f6cc" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019e4114-b67f-7496-b634-8eb6ea352168" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019e4114-b67f-7496-b634-8eb6ea352168" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ShareBasedCompensationArrangementByShareBasedPaymentAwardPurchasePeriod_019e4114-b67f-7025-80e2-4c7f5487407c" xlink:href="upst-20260630.xsd#upst_ShareBasedCompensationArrangementByShareBasedPaymentAwardPurchasePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_upst_ShareBasedCompensationArrangementByShareBasedPaymentAwardPurchasePeriod_019e4114-b67f-7025-80e2-4c7f5487407c" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019e4114-b67f-7616-9506-a6103d43fb72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019e4114-b67f-7616-9506-a6103d43fb72" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e4114-b67f-7959-b782-e7e1d4b18ca8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e4114-b67f-7959-b782-e7e1d4b18ca8" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e4114-b67f-7857-b03c-d9067ce8e158" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7b62-8025-0bbedc33a8eb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e4114-b67f-7857-b03c-d9067ce8e158" xlink:type="arc" order="21"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/StockholdersEquityScheduleofReservedSharesofCommonStockforIssuanceDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#StockholdersEquityScheduleofReservedSharesofCommonStockforIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/StockholdersEquityScheduleofReservedSharesofCommonStockforIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e4114-b67f-74a9-8d7f-37d763f2da30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e4114-b67f-768b-a955-1cc1d78c60cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4114-b67f-74a9-8d7f-37d763f2da30" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e4114-b67f-768b-a955-1cc1d78c60cf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7585-8266-bbde021644de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e4114-b67f-768b-a955-1cc1d78c60cf" xlink:to="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7585-8266-bbde021644de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7585-8266-bbde021644de_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7585-8266-bbde021644de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7585-8266-bbde021644de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-710e-b709-9304292ab3d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7585-8266-bbde021644de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-710e-b709-9304292ab3d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-7d24-bee3-24cbfc381b3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-710e-b709-9304292ab3d8" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-7d24-bee3-24cbfc381b3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-7da6-bca9-9cbc6d3d1456" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-710e-b709-9304292ab3d8" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-7da6-bca9-9cbc6d3d1456" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-7abf-bf75-b9fe5972f185" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-710e-b709-9304292ab3d8" xlink:to="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-7abf-bf75-b9fe5972f185" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7f17-9082-b24de0d903f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-710e-b709-9304292ab3d8" xlink:to="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7f17-9082-b24de0d903f9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e4114-b67f-736f-a03a-63b0bab03cad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e4114-b67f-768b-a955-1cc1d78c60cf" xlink:to="loc_us-gaap_PlanNameAxis_019e4114-b67f-736f-a03a-63b0bab03cad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e4114-b67f-736f-a03a-63b0bab03cad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e4114-b67f-736f-a03a-63b0bab03cad" xlink:to="loc_us-gaap_PlanNameDomain_019e4114-b67f-736f-a03a-63b0bab03cad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e4114-b67f-759b-8c30-38a7cd9d2640" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e4114-b67f-736f-a03a-63b0bab03cad" xlink:to="loc_us-gaap_PlanNameDomain_019e4114-b67f-759b-8c30-38a7cd9d2640" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_A2020EquityIncentivePlanMember_019e4114-b67f-7654-89f0-bc764c59006d" xlink:href="upst-20260630.xsd#upst_A2020EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e4114-b67f-759b-8c30-38a7cd9d2640" xlink:to="loc_upst_A2020EquityIncentivePlanMember_019e4114-b67f-7654-89f0-bc764c59006d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e4114-b67f-76d5-9ab0-27100c1d566f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e4114-b67f-768b-a955-1cc1d78c60cf" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e4114-b67f-76d5-9ab0-27100c1d566f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4114-b67f-76d5-9ab0-27100c1d566f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4114-b67f-76d5-9ab0-27100c1d566f" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4114-b67f-76d5-9ab0-27100c1d566f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4114-b67f-7a6d-ad48-b78a3d8f0490" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4114-b67f-76d5-9ab0-27100c1d566f" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4114-b67f-7a6d-ad48-b78a3d8f0490" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e4114-b67f-72eb-8d1e-1717c0ddb642" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4114-b67f-7a6d-ad48-b78a3d8f0490" xlink:to="loc_us-gaap_CommonClassAMember_019e4114-b67f-72eb-8d1e-1717c0ddb642" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e4114-b67f-78b5-b456-0a1b99f1d768" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4114-b67f-74a9-8d7f-37d763f2da30" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e4114-b67f-78b5-b456-0a1b99f1d768" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/StockholdersEquityScheduleofRSUandPRSUActivityDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#StockholdersEquityScheduleofRSUandPRSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/StockholdersEquityScheduleofRSUandPRSUActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7444-ade8-b376c9001a2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7c37-ae04-671f233ed717" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7444-ade8-b376c9001a2c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7c37-ae04-671f233ed717" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7de5-b3a4-af056fd50a7e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7c37-ae04-671f233ed717" xlink:to="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7de5-b3a4-af056fd50a7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7de5-b3a4-af056fd50a7e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7de5-b3a4-af056fd50a7e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7de5-b3a4-af056fd50a7e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7ae8-9d4f-838e47f6cab9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-7de5-b3a4-af056fd50a7e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7ae8-9d4f-838e47f6cab9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-7bba-ae18-7155f153dbc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7ae8-9d4f-838e47f6cab9" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-7bba-ae18-7155f153dbc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-7b77-9d88-d0fe976714d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7ae8-9d4f-838e47f6cab9" xlink:to="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-7b77-9d88-d0fe976714d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4114-b67f-741f-8a45-c116ea95241e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7444-ade8-b376c9001a2c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4114-b67f-741f-8a45-c116ea95241e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e4114-b67f-705b-9fe5-22a419cda63c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4114-b67f-741f-8a45-c116ea95241e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e4114-b67f-705b-9fe5-22a419cda63c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e4114-b67f-75d2-8bfd-f8e34875c215" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4114-b67f-741f-8a45-c116ea95241e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e4114-b67f-75d2-8bfd-f8e34875c215" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e4114-b67f-78c0-83ea-cb040759ae98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4114-b67f-741f-8a45-c116ea95241e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e4114-b67f-78c0-83ea-cb040759ae98" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e4114-b67f-787b-a9b4-b7088d31ab05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4114-b67f-741f-8a45-c116ea95241e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e4114-b67f-787b-a9b4-b7088d31ab05" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e4114-b67f-7ea8-bd81-5439c60568cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4114-b67f-7eb1-baee-97ccaeaa56ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7444-ade8-b376c9001a2c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4114-b67f-7eb1-baee-97ccaeaa56ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e4114-b67f-7709-8069-be37240ef145" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4114-b67f-7eb1-baee-97ccaeaa56ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e4114-b67f-7709-8069-be37240ef145" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e4114-b67f-7d39-aa86-ece6bab24457" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4114-b67f-7eb1-baee-97ccaeaa56ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e4114-b67f-7d39-aa86-ece6bab24457" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e4114-b67f-7c31-949e-dfc27c0640cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4114-b67f-7eb1-baee-97ccaeaa56ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e4114-b67f-7c31-949e-dfc27c0640cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e4114-b67f-77fb-9a5b-bdf49331abba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4114-b67f-7eb1-baee-97ccaeaa56ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e4114-b67f-77fb-9a5b-bdf49331abba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e4114-b67f-7ae2-9fb0-ce9411022edb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/StockholdersEquityScheduleofWeightedAverageAssumptionsDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#StockholdersEquityScheduleofWeightedAverageAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/StockholdersEquityScheduleofWeightedAverageAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7ee2-9b68-4a5989f0c1a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7ee2-9b68-4a5989f0c1a7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e4114-b67f-737f-936d-1a53caae1f0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7ee2-9b68-4a5989f0c1a7" xlink:to="loc_us-gaap_AwardTypeAxis_019e4114-b67f-737f-936d-1a53caae1f0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-737f-936d-1a53caae1f0b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-737f-936d-1a53caae1f0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-737f-936d-1a53caae1f0b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7063-8d23-8bdec994597d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e4114-b67f-737f-936d-1a53caae1f0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7063-8d23-8bdec994597d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-7f83-bcd4-32ee2883bf83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7063-8d23-8bdec994597d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-7f83-bcd4-32ee2883bf83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7893-89eb-e3bfaf938a02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7063-8d23-8bdec994597d" xlink:to="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7893-89eb-e3bfaf938a02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-71bf-b03f-f56d2c0f3975" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4114-b67f-7063-8d23-8bdec994597d" xlink:to="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-71bf-b03f-f56d2c0f3975" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e4114-b67f-7f03-993a-ec88bed0ee26" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4114-b67f-7ee2-9b68-4a5989f0c1a7" xlink:to="loc_srt_RangeAxis_019e4114-b67f-7f03-993a-ec88bed0ee26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-7f03-993a-ec88bed0ee26_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e4114-b67f-7f03-993a-ec88bed0ee26" xlink:to="loc_srt_RangeMember_019e4114-b67f-7f03-993a-ec88bed0ee26_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e4114-b67f-7a2d-9ffd-69d516840cee" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e4114-b67f-7f03-993a-ec88bed0ee26" xlink:to="loc_srt_RangeMember_019e4114-b67f-7a2d-9ffd-69d516840cee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e4114-b67f-74d8-9279-40235fc3f3e2" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7a2d-9ffd-69d516840cee" xlink:to="loc_srt_MinimumMember_019e4114-b67f-74d8-9279-40235fc3f3e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e4114-b67f-7ad7-87c1-83499332aea5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e4114-b67f-7a2d-9ffd-69d516840cee" xlink:to="loc_srt_MaximumMember_019e4114-b67f-7ad7-87c1-83499332aea5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e4114-b67f-7760-b2c4-72a51074a55e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e4114-b67f-7760-b2c4-72a51074a55e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_019e4114-b67f-7240-99b7-1217a18d4910" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_019e4114-b67f-7240-99b7-1217a18d4910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_019e4114-b67f-74a4-96cb-7040c2ceca16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_019e4114-b67f-74a4-96cb-7040c2ceca16" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e4114-b67f-7a26-b81b-3c89545c762f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e4114-b67f-7a26-b81b-3c89545c762f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e4114-b67f-7a11-9d92-ae6d222fcffb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e4114-b67f-7a11-9d92-ae6d222fcffb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_019e4114-b67f-7c76-b738-c85050aed3b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_019e4114-b67f-7c76-b738-c85050aed3b2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_019e4114-b67f-7ce8-9a5b-89c61cec673f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_019e4114-b67f-7ce8-9a5b-89c61cec673f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e4114-b67f-7801-9ec5-6934d8c49a94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e4114-b67f-7801-9ec5-6934d8c49a94" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f60a4-f693-7d6e-a37d-c44814eb3488" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f60a4-f693-7d6e-a37d-c44814eb3488" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f60a7-6d66-760d-b4b9-dad58fc5d0f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4114-b67f-7854-90d6-a6da45495d40" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f60a7-6d66-760d-b4b9-dad58fc5d0f5" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/StockholdersEquityScheduleofStockbasedCompensationExpenseDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#StockholdersEquityScheduleofStockbasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/StockholdersEquityScheduleofStockbasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e4114-b67f-7675-8e73-97d92abe077f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e4114-b67f-76c9-943d-5be353d0cb83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e4114-b67f-7675-8e73-97d92abe077f" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e4114-b67f-76c9-943d-5be353d0cb83" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e4114-b67f-797c-b0f4-b121ab616913" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e4114-b67f-76c9-943d-5be353d0cb83" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e4114-b67f-797c-b0f4-b121ab616913" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4114-b67f-72b0-b92d-0bd49fff47b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e4114-b67f-7675-8e73-97d92abe077f" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4114-b67f-72b0-b92d-0bd49fff47b4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/StockholdersEquityScheduleofStockbasedCompensationExpenseDetails_1" xlink:type="simple" xlink:href="upst-20260630.xsd#StockholdersEquityScheduleofStockbasedCompensationExpenseDetails_1"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/StockholdersEquityScheduleofStockbasedCompensationExpenseDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e4114-b67f-7675-8e73-97d92abe077f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e4114-b67f-76c9-943d-5be353d0cb83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e4114-b67f-7675-8e73-97d92abe077f" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e4114-b67f-76c9-943d-5be353d0cb83" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4114-b67f-72b0-b92d-0bd49fff47b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e4114-b67f-7675-8e73-97d92abe077f" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4114-b67f-72b0-b92d-0bd49fff47b4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/LeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f8553-a02e-7554-8564-13a903d4ad33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019f8553-a02e-791d-bc4d-804c8bc32259" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f8553-a02e-7554-8564-13a903d4ad33" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019f8553-a02e-791d-bc4d-804c8bc32259" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_019f8553-a02e-72d8-84a3-97467451cc1d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019f8553-a02e-791d-bc4d-804c8bc32259" xlink:to="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_019f8553-a02e-72d8-84a3-97467451cc1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_019f8553-a02e-72d8-84a3-97467451cc1d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_019f8553-a02e-72d8-84a3-97467451cc1d" xlink:to="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_019f8553-a02e-72d8-84a3-97467451cc1d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_019f8553-a02e-78e6-8a69-fb4c89cb2dc0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_019f8553-a02e-72d8-84a3-97467451cc1d" xlink:to="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_019f8553-a02e-78e6-8a69-fb4c89cb2dc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OfficeBuildingMember_019f8554-25e9-7d41-bc10-a4eb434fc3c0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OfficeBuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_019f8553-a02e-78e6-8a69-fb4c89cb2dc0" xlink:to="loc_srt_OfficeBuildingMember_019f8554-25e9-7d41-bc10-a4eb434fc3c0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_019f3788-303f-7e23-9d37-7e0124392bec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f8553-a02e-7554-8564-13a903d4ad33" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_019f3788-303f-7e23-9d37-7e0124392bec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_019f3789-0384-799c-b1a1-5999d9d01a61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f8553-a02e-7554-8564-13a903d4ad33" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_019f3789-0384-799c-b1a1-5999d9d01a61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_019e4114-b67f-7b4c-a776-cf1e9a7250b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f8553-a02e-7554-8564-13a903d4ad33" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_019e4114-b67f-7b4c-a776-cf1e9a7250b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_019e4114-b67f-7889-857e-12f92c1b0075" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f8553-a02e-7554-8564-13a903d4ad33" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_019e4114-b67f-7889-857e-12f92c1b0075" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019e4114-b67f-7cca-8236-7da28441a403" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:to="loc_us-gaap_LossContingenciesTable_019e4114-b67f-7cca-8236-7da28441a403" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019e4114-b67f-78a0-9fa7-3863307700af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e4114-b67f-7cca-8236-7da28441a403" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019e4114-b67f-78a0-9fa7-3863307700af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_019e4114-b67f-78a0-9fa7-3863307700af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019e4114-b67f-78a0-9fa7-3863307700af" xlink:to="loc_us-gaap_LossContingencyNatureDomain_019e4114-b67f-78a0-9fa7-3863307700af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_019e4114-b67f-78b4-b285-8f295e68f1d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019e4114-b67f-78a0-9fa7-3863307700af" xlink:to="loc_us-gaap_LossContingencyNatureDomain_019e4114-b67f-78b4-b285-8f295e68f1d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ObligationToRepurchaseReceivablesSoldMember_019e4114-b67f-7c18-bdb5-351e8f74a68e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ObligationToRepurchaseReceivablesSoldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_019e4114-b67f-78b4-b285-8f295e68f1d9" xlink:to="loc_us-gaap_ObligationToRepurchaseReceivablesSoldMember_019e4114-b67f-7c18-bdb5-351e8f74a68e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7993-aa16-71db21d58618" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e4114-b67f-7cca-8236-7da28441a403" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7993-aa16-71db21d58618" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7993-aa16-71db21d58618_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7993-aa16-71db21d58618" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7993-aa16-71db21d58618_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7b9d-a1e6-37fb020da2e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019e4114-b67f-7993-aa16-71db21d58618" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7b9d-a1e6-37fb020da2e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HomeEquityMember_019e4114-b67f-766f-a126-b67896cf1660" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HomeEquityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019e4114-b67f-7b9d-a1e6-37fb020da2e4" xlink:to="loc_us-gaap_HomeEquityMember_019e4114-b67f-766f-a126-b67896cf1660" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_LossContingencyOwnershipLoanFacilitatedTerm_019e4114-b67f-7ea8-b533-f0d36f4e0b4f" xlink:href="upst-20260630.xsd#upst_LossContingencyOwnershipLoanFacilitatedTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:to="loc_upst_LossContingencyOwnershipLoanFacilitatedTerm_019e4114-b67f-7ea8-b533-f0d36f4e0b4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligation_019e4114-b67f-72ad-bc6a-47e2f97efdd1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PurchaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:to="loc_us-gaap_PurchaseObligation_019e4114-b67f-72ad-bc6a-47e2f97efdd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligation_019e4114-b67f-7d8a-84c1-6e0b2169f2d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractualObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:to="loc_us-gaap_ContractualObligation_019e4114-b67f-7d8a-84c1-6e0b2169f2d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_019e4114-b67f-7a96-821b-ed87f1982e15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyEstimateOfPossibleLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:to="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_019e4114-b67f-7a96-821b-ed87f1982e15" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesPaidValue_019e4114-b67f-7558-aa9d-4d9d4258f761" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyDamagesPaidValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e4114-b67f-7e3a-be7e-16f247d69ade" xlink:to="loc_us-gaap_LossContingencyDamagesPaidValue_019e4114-b67f-7558-aa9d-4d9d4258f761" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/NetIncomePerShareScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsPerShareDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#NetIncomePerShareScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/NetIncomePerShareScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e4114-b67f-779d-82a9-b817fb7b309f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e4114-b67f-764d-a6e1-ba488433ad7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e4114-b67f-779d-82a9-b817fb7b309f" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e4114-b67f-764d-a6e1-ba488433ad7d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4114-b67f-7318-a4af-340a8cb9efa7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e4114-b67f-764d-a6e1-ba488433ad7d" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4114-b67f-7318-a4af-340a8cb9efa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-7318-a4af-340a8cb9efa7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4114-b67f-7318-a4af-340a8cb9efa7" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-7318-a4af-340a8cb9efa7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4114-b67f-7318-a4af-340a8cb9efa7" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-733a-ac2a-c5746fde7a2e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e4114-b67f-733a-ac2a-c5746fde7a2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-72e1-951b-20deb5b2d1e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4114-b67f-72e1-951b-20deb5b2d1e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-7de5-b4f3-71cad2a2642f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:to="loc_us-gaap_PerformanceSharesMember_019e4114-b67f-7de5-b4f3-71cad2a2642f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7aef-a2ca-bb365e6d106d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:to="loc_us-gaap_EmployeeStockMember_019e4114-b67f-7aef-a2ca-bb365e6d106d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_019e4114-b67f-7285-b2de-9e69430e1a7c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4114-b67f-722b-a315-16e69250a2ae" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_019e4114-b67f-7285-b2de-9e69430e1a7c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e4114-b67f-76c4-a019-c64bf9af02bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e4114-b67f-779d-82a9-b817fb7b309f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e4114-b67f-76c4-a019-c64bf9af02bd" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/SegmentInformationScheduleofContributionProfitfortheCompanysUnsecuredLendingSegmentDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#SegmentInformationScheduleofContributionProfitfortheCompanysUnsecuredLendingSegmentDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/SegmentInformationScheduleofContributionProfitfortheCompanysUnsecuredLendingSegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b67f-796f-a2d2-e411d0c519fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-7e47-9161-b3362483ddbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b67f-796f-a2d2-e411d0c519fa" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-7e47-9161-b3362483ddbd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-7059-b636-cde903c8c9ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-7e47-9161-b3362483ddbd" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-7059-b636-cde903c8c9ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e4114-b67f-7059-b636-cde903c8c9ac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-7059-b636-cde903c8c9ac" xlink:to="loc_us-gaap_SegmentDomain_019e4114-b67f-7059-b636-cde903c8c9ac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e4114-b67f-7d8e-ae6e-bf39afefb807" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-7059-b636-cde903c8c9ac" xlink:to="loc_us-gaap_SegmentDomain_019e4114-b67f-7d8e-ae6e-bf39afefb807" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_UnsecuredLendingSegmentMember_019e4114-b67f-7d3e-b8fb-6f26276ad1e5" xlink:href="upst-20260630.xsd#upst_UnsecuredLendingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e4114-b67f-7d8e-ae6e-bf39afefb807" xlink:to="loc_upst_UnsecuredLendingSegmentMember_019e4114-b67f-7d3e-b8fb-6f26276ad1e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_RevenueFromFeesNet_019e4114-b67f-7387-a13b-9d260df9240e" xlink:href="upst-20260630.xsd#upst_RevenueFromFeesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b67f-796f-a2d2-e411d0c519fa" xlink:to="loc_upst_RevenueFromFeesNet_019e4114-b67f-7387-a13b-9d260df9240e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BorrowerAcquisitionCosts_019e4114-b67f-72cd-ab46-0fc70d3f4037" xlink:href="upst-20260630.xsd#upst_BorrowerAcquisitionCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b67f-796f-a2d2-e411d0c519fa" xlink:to="loc_upst_BorrowerAcquisitionCosts_019e4114-b67f-72cd-ab46-0fc70d3f4037" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BorrowerVerificationAndServicingCosts_019e4114-b67f-7522-b12f-2e31ece3a8c8" xlink:href="upst-20260630.xsd#upst_BorrowerVerificationAndServicingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b67f-796f-a2d2-e411d0c519fa" xlink:to="loc_upst_BorrowerVerificationAndServicingCosts_019e4114-b67f-7522-b12f-2e31ece3a8c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ContributionProfit_019e4114-b67f-740d-ab1f-eaa53a296bab" xlink:href="upst-20260630.xsd#upst_ContributionProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b67f-796f-a2d2-e411d0c519fa" xlink:to="loc_upst_ContributionProfit_019e4114-b67f-740d-ab1f-eaa53a296bab" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.upstartholdings.com/role/SegmentInformationScheduleofReconciliationofTotalContributionProfittoNetLossBeforeIncomeTaxesDetails" xlink:type="simple" xlink:href="upst-20260630.xsd#SegmentInformationScheduleofReconciliationofTotalContributionProfittoNetLossBeforeIncomeTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.upstartholdings.com/role/SegmentInformationScheduleofReconciliationofTotalContributionProfittoNetLossBeforeIncomeTaxesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-77b8-98eb-259f71471da7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-77b8-98eb-259f71471da7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-714c-aaa4-d44fb679cac0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-77b8-98eb-259f71471da7" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-714c-aaa4-d44fb679cac0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e4114-b67f-714c-aaa4-d44fb679cac0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-714c-aaa4-d44fb679cac0" xlink:to="loc_us-gaap_SegmentDomain_019e4114-b67f-714c-aaa4-d44fb679cac0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e4114-b67f-7974-9c7a-dd5e4ed7ef66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e4114-b67f-714c-aaa4-d44fb679cac0" xlink:to="loc_us-gaap_SegmentDomain_019e4114-b67f-7974-9c7a-dd5e4ed7ef66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_UnsecuredLendingSegmentMember_019e4114-b67f-74bd-8ddf-4ef113164187" xlink:href="upst-20260630.xsd#upst_UnsecuredLendingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e4114-b67f-7974-9c7a-dd5e4ed7ef66" xlink:to="loc_upst_UnsecuredLendingSegmentMember_019e4114-b67f-74bd-8ddf-4ef113164187" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e4114-b67f-7192-8b03-64018aba7a2d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e4114-b67f-77b8-98eb-259f71471da7" xlink:to="loc_srt_ConsolidationItemsAxis_019e4114-b67f-7192-8b03-64018aba7a2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e4114-b67f-7192-8b03-64018aba7a2d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e4114-b67f-7192-8b03-64018aba7a2d" xlink:to="loc_srt_ConsolidationItemsDomain_019e4114-b67f-7192-8b03-64018aba7a2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e4114-b67f-7913-9253-50223bd29fe6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e4114-b67f-7192-8b03-64018aba7a2d" xlink:to="loc_srt_ConsolidationItemsDomain_019e4114-b67f-7913-9253-50223bd29fe6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e4114-b749-73ff-ac34-e3872b6aec09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e4114-b67f-7913-9253-50223bd29fe6" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e4114-b749-73ff-ac34-e3872b6aec09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ContributionProfit_019e4114-b749-7f7d-857d-b15efbc3e13b" xlink:href="upst-20260630.xsd#upst_ContributionProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_upst_ContributionProfit_019e4114-b749-7f7d-857d-b15efbc3e13b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ContributionProfitOther_019e4114-b749-7986-9479-c1703f22deab" xlink:href="upst-20260630.xsd#upst_ContributionProfitOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_upst_ContributionProfitOther_019e4114-b749-7986-9479-c1703f22deab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_019e4114-b749-791a-8e22-61d5305be2aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_SellingAndMarketingExpense_019e4114-b749-791a-8e22-61d5305be2aa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_CustomerOperationsExpense_019e4114-b749-7787-8b75-a42cf4501ac6" xlink:href="upst-20260630.xsd#upst_CustomerOperationsExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_upst_CustomerOperationsExpense_019e4114-b749-7787-8b75-a42cf4501ac6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019e4114-b749-7128-bee4-6299a01cc385" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019e4114-b749-7128-bee4-6299a01cc385" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019e4114-b749-7d10-832b-fcc635e7c3de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019e4114-b749-7d10-832b-fcc635e7c3de" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNet_019e4114-b749-7c22-b7b7-6e95e7354aeb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_InterestIncomeExpenseNet_019e4114-b749-7c22-b7b7-6e95e7354aeb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e4114-b749-71ac-ba5d-cfc0f284bf4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e4114-b749-71ac-ba5d-cfc0f284bf4a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossAttributableToParent_019e4114-b749-73ba-b358-3fd80a75ddfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_us-gaap_IncomeLossAttributableToParent_019e4114-b749-73ba-b358-3fd80a75ddfd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BorrowerAcquisitionCosts_019e4114-b749-759b-a3a3-0be06cb144b3" xlink:href="upst-20260630.xsd#upst_BorrowerAcquisitionCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_upst_BorrowerAcquisitionCosts_019e4114-b749-759b-a3a3-0be06cb144b3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_BorrowerVerificationAndServicingCosts_019e4114-b749-71b7-87ad-1cbc8979ac35" xlink:href="upst-20260630.xsd#upst_BorrowerVerificationAndServicingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e4114-b749-7061-9836-1b4574ad805b" xlink:to="loc_upst_BorrowerVerificationAndServicingCosts_019e4114-b749-71b7-87ad-1cbc8979ac35" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_AndreaBlankmeyerMember_019f379f-0966-75cb-9e97-5a9ef9d3e1d3" xlink:href="upst-20260630.xsd#upst_AndreaBlankmeyerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_upst_AndreaBlankmeyerMember_019f379f-0966-75cb-9e97-5a9ef9d3e1d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_ScottDarlingMember_019e4114-b1ea-7c3f-ac70-b0313decd1d8" xlink:href="upst-20260630.xsd#upst_ScottDarlingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_upst_ScottDarlingMember_019e4114-b1ea-7c3f-ac70-b0313decd1d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_upst_NataliaMirgorodskayaMember_019e4114-b1ea-75b8-86b2-adc3e5d451f3" xlink:href="upst-20260630.xsd#upst_NataliaMirgorodskayaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_upst_NataliaMirgorodskayaMember_019e4114-b1ea-75b8-86b2-adc3e5d451f3" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
