This book constitutes the thoroughly refereed post-conference proceedings of the 20th International Workshop on Multi-Agent-Based Simulation, MABS 2019, held in Montreal, QC, Canada, in May 2019 as part of the AAMAS 2019, the 18th International Conference on Autonomous Agents and Multiagent Systems. The 9 revised full papers included in this volume were carefully selected from 15 submissions. They focus on finding efficient solutions to model complex social systems in such areas as economics, management, and organisational and social sciences. In all these areas, agent theories, metaphors, models, analysis, experimental designs, empirical studies, and methodological principles, converge into simulation as a way of achieving explanations and predictions, exploration and testing of hypotheses, better designs and systems.