More than half of the total population in China are living in cities. Especially, the people in highly developed and spatially integrated city clusters, i.e., urban agglomerations (UAs), are facing increasing human‐perceived heat stress that describes the combined effects of hot temperature, high humidity, and lowered surface wind speed. By analyzing multiple indicators over 20 major UAs across China, we demonstrate that summer heat stress has been significantly intensifying in nearly all UAs during 1971–2014. This intensification is more profound in northern than southern regions and is especially stronger in more urbanized and densely populated areas (e.g., Beijing‐Tianjin‐Hebei and the Yangtze River Delta). Based on a dynamic classification of weather stations using time‐varying land use/land cover maps, we find that urban core areas exhibit distinctly stronger increasing heat stress trends than their surrounding rural areas. On average, urbanization contributes to approximately one‐quarter of the total increase in mean heat stress over urban core areas of UAs and nearly half of the total increase in extreme heat stress events. The urbanization effect is also dependent on the geographical region within China. Urbanization tends to have stronger intensifying effects on heat stress in UAs with higher population density in low‐altitude areas, while it has a relatively weaker intensifying and even weakening effect in some arid and high‐altitude regions. Moreover, as various heat stress metrics may yield different estimations of long‐term trend and urbanization contribution, the particular choice of heat stress indicator is of critical importance for investigations on this subject matter. Plain Language Summary: More and more people are living in highly developed and spatially integrated city clusters (known as urban agglomerations, UAs) and facing increasing heat risks in a warming climate. Human‐perceived heat stress describes the combined effects of hot temperature, high humidity, and lowered surface wind speed, posing severe threats to human society and the natural environment. However, how heat stress changes in UAs of fast‐urbanizing China and to what degree local urbanization contributes to these changes remains unclear. Here, we investigate the changes of heat stress over the largest 20 UAs across China by examining eight different heat stress indicators and quantify the relative contribution of local urbanization to these changes. We show that all indicators of heat stress increase in nearly all UAs of China, especially in more urbanized and populated UAs. It is estimated that, on average, local urbanization contributes to around one‐quarter of the total increase in mean heat stress in the urban core areas of UA and nearly half of the increase in extreme heat stress events. We also find that urbanization tends to have a stronger warming effect on wet and low‐altitude regions and relative weaker warming or event cooling effects on arid and high‐altitude areas. Key Points: All indicators of heat stress increase in nearly all major urban agglomerations (UAs), especially in more urbanized and populated UAsOn average, local urbanization contributes to ≈1/4 (1/2) of the total increase in mean (extreme) heat stress in the urban core areas of UAsUrbanization tends to have a strong warming (cooling or weak warming) effect on heat stress in wet (arid) areas and low (high) altitudes [ABSTRACT FROM AUTHOR]