This book constitutes the thoroughly refereed post-conference proceedings of the 9th International Conference on Fundamentals of Software Engineering, FSEN 2021, held virtually and hosted by IPM in May 2021. The 12 full papers and 4 short papers presented in this volume were carefully reviewed and selected from 38 submissions. The topics of interest in FSEN span over all aspects of formal methods, especially those related to advancing the application of formal methods in the software industry and promoting their integration with practical engineering techniques. The papers are organized in topical sections on coordination, logic, networks, parallel computation, and testing.