This book constitutes the refereed proceedings of the 21st International Conference on Web and Internet Economics, WINE 2025, held in New Brunswick, NJ, USA, during December 8-11, 2025.

The 39 full papers and 28 abstracts included in this volume were carefully reviewed and selected from 225 submissions. They are organized into the following topical sections: Mechanism Design and Pricing; Matching, Markets, and Equilibria; Fairness and Social Choice; AI, Blockchain, and Game Theory; and Online Algorithms.