Climate change increases the frequency and intensity of heatwaves, amplifying heat-related health risks in cities worldwide. Inequities in heat vulnerability arise from disparities in heat exposure, built and natural environments and population attributes that impact heat sensitivity, and socio-economic determinants of adaptive capability. A lack of internationally consistent and accessible heat vulnerability metrics creates barriers to assessing inequities and benchmarking urban heat vulnerability between cities worldwide. To address this need, we developed the Global Urban Heat Vulnerability Index (GUHVI), applicable to cities worldwide, using open data to identify spatial inequities in heat vulnerability at the neighbourhood scale. Built from an Australia-specific heat vulnerability index, the evidence-informed framework developed for the GUHVI evaluates heat exposure, heat sensitivity and adaptive capability to holistically assess heat vulnerability. Quantitative validation for eight Australian cities demonstrated strengths of the GUHVI in spatial resolution and assessment coverage of the grid-based framework. Qualitative validation for nine diverse cities internationally was performed in collaboration with local subject matter experts with knowledge of each city context. The GUHVI addresses critical gaps in existing methods by enabling systematic and comparable measurement of heat vulnerability in diverse cities internationally. Available through our customizable open-source global indicator software, the GUHVI provides evidence on modifiable risk factors of urban heat vulnerability, to inform targeted adaptation strategies that promote climate resilience and reduce health impacts from heat.