This book constitutes the refereed proceedings of the 20th International Conference on Critical Information Infrastructures Security, CRITIS 2025, held in Jönköping, Sweden, during October 21–23, 2025. The 22 full papers presented in this volume were carefully reviewed and selected from 38 submissions. They were grouped into the following topics: Methods & Practice; Power Grids; Security and Safety Large and Small; Vehicles; Methods; Knowledge, Skills and Education; AI and Critical Infrastructures; Security Frameworks.