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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019eb6a6-2bf1-73fc-8c65-9701dc619387,g:d9911086-0c6d-444a-9352-fdb2007120f5-->
<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.summitplc.com/role/Cover" xlink:type="simple" xlink:href="smmt-20260610.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesTable_019eb6a6-2bf1-7793-823f-34d1dc457980" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityAddressesTable_019eb6a6-2bf1-7793-823f-34d1dc457980" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesAddressTypeAxis_019eb6a6-2bf1-7098-b541-fe2542c94fa8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressesAddressTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntityAddressesTable_019eb6a6-2bf1-7793-823f-34d1dc457980" xlink:to="loc_dei_EntityAddressesAddressTypeAxis_019eb6a6-2bf1-7098-b541-fe2542c94fa8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_019eb6a6-2bf1-7098-b541-fe2542c94fa8_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_019eb6a6-2bf1-7098-b541-fe2542c94fa8" xlink:to="loc_dei_AddressTypeDomain_019eb6a6-2bf1-7098-b541-fe2542c94fa8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_019eb6a6-2bf1-7b72-b780-f4f10a1202c1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_019eb6a6-2bf1-7098-b541-fe2542c94fa8" xlink:to="loc_dei_AddressTypeDomain_019eb6a6-2bf1-7b72-b780-f4f10a1202c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_FormerAddressMember_019eb6a6-2bf1-7063-9039-a74b9040a6b6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_FormerAddressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_AddressTypeDomain_019eb6a6-2bf1-7b72-b780-f4f10a1202c1" xlink:to="loc_dei_FormerAddressMember_019eb6a6-2bf1-7063-9039-a74b9040a6b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019eb6a6-2bf1-743a-8332-0a62e3caf349" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_DocumentType_019eb6a6-2bf1-743a-8332-0a62e3caf349" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019eb6a6-2bf1-75f9-8bd0-fd7434cb7ffb" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_DocumentPeriodEndDate_019eb6a6-2bf1-75f9-8bd0-fd7434cb7ffb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019eb6a6-2bf1-707a-814c-cf896c8842d8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityRegistrantName_019eb6a6-2bf1-707a-814c-cf896c8842d8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019eb6a6-2bf1-7017-b29a-662a7fba731d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019eb6a6-2bf1-7017-b29a-662a7fba731d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019eb6a6-2bf1-72de-a88e-988786627474" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityFileNumber_019eb6a6-2bf1-72de-a88e-988786627474" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019eb6a6-2bf1-7725-876d-9458c3a813d3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityTaxIdentificationNumber_019eb6a6-2bf1-7725-876d-9458c3a813d3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019eb6a6-2bf1-7f57-9309-f0a237c7e826" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityAddressAddressLine1_019eb6a6-2bf1-7f57-9309-f0a237c7e826" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019eb6a6-2bf1-7a2d-88ca-6283c11069cb" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityAddressCityOrTown_019eb6a6-2bf1-7a2d-88ca-6283c11069cb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019eb6a6-2bf1-7759-a2c8-6e314c63cad7" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityAddressStateOrProvince_019eb6a6-2bf1-7759-a2c8-6e314c63cad7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019eb6a6-2bf1-7b9e-bc7a-a66ffdc56085" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityAddressPostalZipCode_019eb6a6-2bf1-7b9e-bc7a-a66ffdc56085" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019eb6a6-2bf1-7931-8b6f-b7ca4b46ff34" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_CityAreaCode_019eb6a6-2bf1-7931-8b6f-b7ca4b46ff34" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019eb6a6-2bf1-7f5d-bfa9-14417b4ce763" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_LocalPhoneNumber_019eb6a6-2bf1-7f5d-bfa9-14417b4ce763" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_019eb6a6-2bf1-763e-b41b-a5d7cee3b800" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_WrittenCommunications_019eb6a6-2bf1-763e-b41b-a5d7cee3b800" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_019eb6a6-2bf1-7ab4-a63f-d220609ce83f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_SolicitingMaterial_019eb6a6-2bf1-7ab4-a63f-d220609ce83f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_019eb6a6-2bf1-7f03-9bb3-31c49e4cd440" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_PreCommencementTenderOffer_019eb6a6-2bf1-7f03-9bb3-31c49e4cd440" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_019eb6a6-2bf1-7cff-8d61-73d38d476612" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_019eb6a6-2bf1-7cff-8d61-73d38d476612" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019eb6a6-2bf1-71e9-af15-64472657648c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_Security12bTitle_019eb6a6-2bf1-71e9-af15-64472657648c" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019eb6a6-2bf1-7bd6-b5b3-c65401c87110" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_TradingSymbol_019eb6a6-2bf1-7bd6-b5b3-c65401c87110" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019eb6a6-2bf1-7c43-8d87-db66d85beef6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_SecurityExchangeName_019eb6a6-2bf1-7c43-8d87-db66d85beef6" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019eb6a6-2bf1-76f5-81d5-329855cfc95f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityEmergingGrowthCompany_019eb6a6-2bf1-76f5-81d5-329855cfc95f" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019eb6a6-2bf1-74a7-9ab6-c2bc1c02ce5e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_EntityCentralIndexKey_019eb6a6-2bf1-74a7-9ab6-c2bc1c02ce5e" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019eb6a6-2bf1-73b6-86db-bae8c7027c09" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_019eb6a6-2bf1-7c05-bc90-e351dfd15bf4" xlink:to="loc_dei_AmendmentFlag_019eb6a6-2bf1-73b6-86db-bae8c7027c09" xlink:type="arc" order="21"/>
  </link:definitionLink>
</link:linkbase>
