This volume constitutes the refereed proceedings of the 21st International Conference on Software and Systems Reuse, ICSR 2024, held in Limassol, Cyprus, during June 19-20, 2024. The 9 full papers presented together with 2 short papers were carefully reviewed and selected from 25 submissions. They focus on the following topics: artificial intelligence and reuse; variability and reuse; adaptation and reuse; and code reuse.