This book constitutes the refereed proceedings of the 20th International Conference on Principles and Practice of Multi-Agent Systems, PRIMA 2017, held in Nice, France, in October/November 2017.  The 24 revised full papers presented together with one abstract of a keynote talk and 11 short papers were carefully reviewed and selected from 88 submissions.  The intention of the papers is to showcase research in several domains, ranging from foundations of agent theory and engineering aspects of agent systems, to emerging interdisciplinary areas of agent-based research.