This book constitutes the refereed proceedings of the 33rd IFIP WG 6.1 International Conference on Testing Software and Systems, ICTSS 2021, which was supposed to be held in London, UK, but was held virtually due to the COVID-19 pandemic. The 10 regular papers and 7 short papers presented were carefully reviewed and selected from 36 submissions. Additionally, the volume includes 6 project reports.  The papers are divided into the following topical subheadings: Finite State Machine-based Testing; Test Generation and Selection; AI-based Techniques; Use Cases; Project Reports.