This volume LNCS 15534 constitutes the refereed proceedings of the 20th International Conference on Web and Internet Economics, WINE 2024, held in Edinburgh, UK, during December 2-5, 2024. The 34 full papers and 32 abstracts included in this volume were carefully reviewed and selected from 248 submissions. These papers cover a wide range of topics such as computational social choice, fair division, resource allocation, pricing, information design, aggregation, blockchains, auctions, matching markets, contracts, games and Nash equilibria, facility location and security.