Urban heat stress is a pressing issue, intensified by climate change and urban heat islands. This study introduces a methodology for creating very-high-resolution districtscale cumulative heat stress maps to support climate-responsive urban planning. Using the SOLWEIG model, we simulated mean radiant temperature (Tmrt) across Barcelona’s Gràcia district, achieving high accuracy by employing an anisotropic sky model and a 10% tree canopy transmissivity coefficient. Simulations for the hottest day of 2022 revealed significant variations in outdoor heat stress due to urban morphology and vegetation. Analysis of three plazas and four urban canyons with differing designs and vegetation levels demonstrated that tree cover can reduce peak Tmrt by over 20°C in exposed areas. Hourly Tmrt maps enabled the creation of a novel district-scale cumulative heat stress map, highlighting daily hours of extreme heat stress and revealing hotspots and cooler zones shaped by urban geometry and tree cover. These findings underscore the critical role of vegetation and urban design in mitigating heat stress and enhancing thermal comfort. The study provides actionable insights for city planners, practitioners, and policymakers to reduce health risks from extreme heat, fostering urban resilience and sustainability amidst global warming.