This book constitutes the refereed proceedings of the 37th IFIP WG 6.1 International Conference on Testing Software and Systems, ICTSS 2025, held in Limassol, Cyprus, during September 17–19, 2025.The 19 full papers and 4 short papers included in this book were carefully reviewed and selected from 38 submissions. They were organized in topical sections as follows: Foundations and Advanced Testing Techniques; Intelligent and Automated Testing; LLMs, Agents, and AI-Driven Testing; Testing in Complex and Security-Critical Systems..