This book constitutes the refereed proceedings of the 25th Nordic Conference on Secure IT Systems, NordSec 2020, which was organized by Linköping University, Sweden, and held online during November 23-24, 2020. The 15 papers presented in this volume were carefully reviewed and selected from 45 submissions. They were organized in topical sections named: malware and attacks; formal analysis; applied cryptography; security mechanisms and training; and applications and privacy.