This book constitutes the refereed proceedings of the 21st International Conference on Software Engineering and Formal Methods, SEFM 2023, held in Eindhoven, The Netherlands, during November 6-10, 2023. The 18 regular papers presented in this book, together with 1 invited paper and 1 tool paper, were carefully reviewed and selected from 41 submissions. The SEFM conference series aims to bring together researchers and practitioners from academia, industry and government, to advance the state of the art in formal methods, to facilitate their uptake in the software industry, and to encourage their integration within practical software engineering methods and tools.