This study presents the first continent-wide assessment of long-term (1974–2023) summer heat stress in Africa using the Universal Thermal Climate Index (UTCI) derived from ERA5-HEAT reanalysis datasets. Employing a grid-specific definition of summer, we analysed UTCI trends across spatial (continental to city scale) and temporal (decadal to hourly) dimensions. While annual UTCI anomalies range 0.2°C–1.6°C, substantial intensification emerges at finer scales. The frequency, duration and spatial extent of ‘very strong’ (38°C–46°C) and ‘extreme’ (≥ 46°C) heat stress have increased markedly, particularly, in the Sahel. Heat-stressed hours (UTCI > 32°C) rose by 2%–25% across most countries, with extreme events doubling in 3–5 and 6–8 days episodes. City-level analyses reveal escalating risks in large urban centres like Cairo, Lagos and Kano. As climate projections indicate further intensification, the findings underscore the urgent need for targeted heat-health adaptation and early warning systems to protect vulnerable populations across the continent.