The two-volume set LNCS 14664 and 14665 constitutes the refereed proceedings of the 21st International Conference on The Semantic Web, ESWC 2024, held in Hersonissos, Crete, Greece, during May 26-30, 2024. The 32 full papers presented were carefully reviewed and selected from 138 submissions. They focus on all aspects of theoretical, analytical, and empirical aspects of the semantic web, semantic technologies, knowledge graphs and semantics on the web in general.