This book constitutes the refereed proceedings of the 26th International Conference on Principles and Practice of Multi-Agent Systems, PRIMA 2025, held in Modena, Italy, during December 16–19, 2025.

The 25 full papers and 20 short papers included in this book were carefully reviewed and selected from 126 submissions.The conference also included three invited talks on cutting-edge topics in Artificial Intelligence and Multi-Agent Systems, presented by: Matteo Baldoni, University of Turin (Italy), Takayuki Ito, Kyoto University (Japan), and Alessio Lomuscio, Imperial College London (United Kingdom).