This book constitutes revised selected papers from the 9th International Workshop on Engineering Multi-Agent Systems, EMAS 2021, which was held during May 3-4, 2021. The conference was initially planned to take place in London, UK, but changed to an online event due to the COVID-19 pandemic. The 20 full papers and 1 short paper included in this volume were carefully reviewed and selected from a total of 27 submissions. The contributions deal with agent-oriented software engineering, programming multi-agent systems, declarative agent languages and technologies, artificial intelligence, and machine learning.