The two-volume set IFIP AICT 745 + 746 constitutes the refereed proceedings of the 40th IFIP International Conference on ICT Systems Security and Privacy Protection, SEC 2025, held in Maribor, Slovenia, during May 21-23, 2025. The 28 full papers and 7 workshop papers included in this book were carefully reviewed and selected from 127 submissions. They were organized in topical sections as follows: Privacy protection; Industrial and Critical Infrastructure Security; Applied Cryptography; Data and Application Security; and International Workshop on Network and Distributed Systems Security (WNDSS 2025).