This book constitutes the proceedings of the 21st International Workshop on Security on Advances in Information and Computer Security, IWSEC 2026, held in Tokyo, Japan, in November 24–26, 2026.

The 17 full papers and 3 short papers presented in this book were carefully reviewed and selected from 52 submissions. These papers were categorized into the following sections: Software, Systems and Cyberphysical Security; Post-Quantum Cryptography; Multiparty Computation and Card-Based Cryptography; AI/ML Security; Symmetric and Homomorphic Cryptography.