This book constitutes the proceedings of the 21st International Workshop on Security and Trust Management, STM 2025, held in conjunction with the 30th European Symposium on Computer Security Research (ESORICS 2025), in Toulouse, France, during September 25–26, 2025. The 13 full papers presented in this book were carefully reviewed and selected from 26 submissions. They were organized in topical sections as follows: Identity Management, Authentication, Access Control & Formal Modeling; AI & Machine Learning for Security; Cybersecurity Strategies, Regulations, Privacy; Cryptography & Threat Analysis.