https://github.com/spring-projects/spring-boot/pull/22258 added a property, spring.security.saml2.relyingparty.*.relying-party-entity-id, for configuring the entity id. It maps onto Spring Security's RelyingPartyRegistration.Builder.entityId(String) method. With hindsight, and thanks to @dawi for the suggestion, spring.security.saml2.relyingparty.*.entityId would be a better name for the property.