This book constitutes the refereed proceedings of the 21st International Conference on Formal Modeling and Analysis of Timed Systems, FORMATS 2023, held in Antwerp, Belgium, in September 2023. The 9 full papers presented in this book were carefully reviewed and selected from 21 submissions. The proceedings also contain one invited paper in full paper length. The papers deal with real-time issues in hardware design, performance analysis, real-time software, scheduling, semantics, and verification of real-timed, hybrid, and probabilistic systems.