Abstract With heat waves becoming more frequent and intense under global warming, heat stress is a growing public health risk. We analyze heat stress trends (Simplified Wet Bulb Globe Temperature, sWBGT) during European heatwaves (1979–2023) using observations and reanalysis data. Results show widespread sWBGT increases, with distinct regional contributions from temperature and relative humidity. In Northern Europe, rising temperature and relative humidity jointly amplify heat stress. In contrast, trends in Western/Central Europe are primarily temperature‐driven, whereas declining humidity partly offsets the temperature‐driven increase in Mediterranean. Interaction and nonlinear residual contributions remain small. Comparisons between ERA5 and station data reveal larger differences in humidity‐driven contributions, particularly in Madrid. Both data sets confirm a sharp rise in alert‐level days, with Madrid experiencing a fivefold increase between the two periods. These findings clarify the spatially varying thermodynamic drivers of heat stress and highlight the expanding burden of extreme heat across temperate and southern Europe.