This book constitutes the refereed proceedings of the 26th International Workshop on Multi-Agent-Based Simulation XXVI, MABS 2025, held in Detroit, MI, USA, on May 19, 2025.The 12 full papers included in this book were carefully reviewed and selected from 20 submissions. They are organized in topical sections as follows: Conceptual and AI-assisted modeling; Public policy and institutions; Markets and engineered systems; and Methods and foundations.