This book constitutes the refereed proceedings of the 15th International Conference on Network and System Security, NSS 2021, held in Tianjin, China, on October 23, 2021. The 16 full and 8 short papers presented in this book were carefully reviewed and selected from 62 submissions. They focus on theoretical and practical aspects of network and system security, such as authentication, access control, availability, integrity, privacy, confidentiality, dependability and sustainability of computer networks and systems.