This book constitutes the proceedings of the 15th International Symposium on Cyberspace Safety and Security, CSS 2025, held in Hangzhou, China, on February 24, 2025.The 28 full papers were presented in this volume were carefully reviewed and selected from 112 submissions. This conference focuses on Cyberspace Safety and Security, such as authentication, access control, availability, integrity, privacy, confidentiality, dependability and sustainability issues of cyberspace.