This book constitutes the refereed proceedings of the 30th Nordic Conference on Secure IT Systems, NordSec 2025, held in Tartu, Estonia, during November 12-13, 2025.The 29 full papers included in this book were carefully reviewed and selected from 89 submissions. They were organized in topical sections as follows: Cryptography; Artificial Intelligence and Software Security; Network and Communication Security; System and Hardware Security; Threat Analysis; Access Control and Policy Management; Usable Security and Societal Resilience; and Obfuscation.