Heatwaves represent one of the most significant climate-related hazards affecting human health, ecosystems, and socioeconomic activities. This study presents the first multi-decadal, national-scale assessment of summer heatwaves and heat vulnerability in Romania using a 26-year archive (2000–2025) of MODIS Land Surface Temperature (LST) integrated with official meteorological warnings and high-resolution population-grid data. Daily satellite observations were processed to analyze long-term trends, anomalies, and consecutive hot days. A 1-km Heat Vulnerability Index (HVI) was developed by fusing satellite-derived thermal hazard data, cumulative heat-warning severity, population density, and demographic sensitivity.</p> <p>The results indicate a severe and widespread intensification of summer thermal conditions across Romania, with August exhibiting the strongest surface warming trend (1.76 °C decade⁻¹), followed by July (0.82 °C decade⁻¹), and a seasonal average increase of 0.84 °C decade⁻¹. Maximum LST frequently exceeded 50 °C in the southern lowlands during the most extreme summers on record (2007, 2012, 2022, 2024, and 2025), with 2024 registering a record-breaking 56 heatwave days. The southern lowlands and the Bucharest metropolitan area emerged as the principal national hotspots of heat-related risk. Nationally, over 3.15 million inhabitants (16.5 % of the total population) reside in areas classified under high and very high vulnerability classes, with Bucharest alone concentrating 1.82 million vulnerable citizens. The proposed framework provides an operational, high-utility tool for geographic screening, directly supporting evidence-based public health interventions and climate adaptation strategies.