This book constitutes the refereed proceedings of the 20th International Workshop on Security and Trust Management (ERCIM STM 2024), held in Bydgoszcz, Poland, during September 19-20, 2024 and co-located with ESORICS 2024 conference. The 6 full papers and 4 short papers presented in this volume were carefully reviewed and selected from 22 submissions. They were organized in topical sections as follows: Trust, Anonymity and Identity; Cryptography, Secure Computation and Formal Methods; Operating Systems and Application Security.