This book constitutes the refereed post-conference proceedings of the 8th International Workshop on Explainable, Trustworthy, and Responsible AI and Multi-Agent Systems, EXTRAAMAS 2026, held in Paphos, Cyprus during May 25–26, 2026.

The 20 full papers and 2 short paper included in this book were carefully reviewed and selected from 34 submissions. They were organized in topical sections as follows: Foundations & Limits of Explainability in Agentic Systems; Trust, Ethics, and Human-Centred Explainable AI; Methods for Explainability: Argumentation, Learning, and Hybrid AI; Systems and Applications of Explainable Multi-Agent Systems.