This book constitutes the refereed proceedings of the 21st European Conference on Multi-Agent Systems, EUMAS 2024, which took place in Dublin, Ireland, on August 26, 2024. The 24 full papers and 1 short paper included in this book were carefully reviewed and selected from 36 submissions. They were organized in topical sections as follows: Multi-Agent Based Simulation; Multi-Agent Learning; Knowledge Representation, Reasoning and Planning; Human-Agent Interaction; Coordination, Organisations, Institutions, Norms and Ethics; and Engineering Multi-Agent Systems.