This book constitutes the refereed proceedings of the 13th International Workshop on Security, IWSEC 2018, held in Sendai, Japan, in September 2018. The 18 regular papers and 2 short papers presented in this volume were carefully reviewed and selected from 64 submissions. They were organized in topical sections named: Cryptanalysis, Implementation Security, Public-Key Primitives, Security in Practice, Secret Sharing, Symmetric-Key Primitives, and Provable Security.