This book constitutes the refereed proceedings of the 27th International Workshop on Multi-Agent-Based Simulation XXVII, MABS 2026, held in Paphos, Cyprus, on May 26, 2026.

The 12 full papers included in this book were carefully reviewed and selected from 27 submissions. They are organized in topical sections as follows: MABS of Social Dynamics; MABS Application to Socio-Ecological Systems; Use of Large Language Models in MABS; and MABS Methodologies and Tools.