This book constitutes the refereed proceedings of the 35th IFIP WG 6.1 International Conference on Testing Software and Systems, ICTSS 2023, held in Bergamo, Italy, during September 18-20, 2023. The 13 full papers presented together with 6 short papers and one journal paper were carefully reviewed and selected from 56 submissions. The conference focuses on Test Case Generation; Test Automation and Design; Model Based Testing; and AI and Smart Contracts Testing.