Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a newly recognized syndrome mediated by anti-platelet factor 4 antibodies induced by Covid-19 adenovirus-vectored vaccines including ChAdOx1 nCoV-19 and Ad26.COV2.S. This study validated a proposed Brighton Collaboration case definition for VITT. A data collection form was developed and used to capture the variations in VITT criteria and assess their level of diagnostic certainty from adjudicated positive VITT case datasheets in Germany (n = 71), UK (n = 220), Australia (n = 203), and Taiwan (n = 56). We observed high prevalence of each component of the proposed VITT definition in positive cases (84%-100%), except for the occurrence of thrombosis or thromboembolism criterion in only 34% of VITT cases in Taiwan. The sensitivity of this proposed definition was 100% for Germany and UK, 92% for Australia, and 89% for Taiwan cases. These findings support the validity of this case definition for VITT.