A combination of hot temperature and high humidity (high heat stress) has severe impacts on environment, society, and public health, especially in urban areas where the majority of the world's population lives. This study investigates the changes of heat stress in urban areas of eastern China and urbanization effects. Data for 242 urban areas and records from a dense network of nearly 2,000 stations are examined. All urban areas have experienced substantial increases in mean heat stress and the frequencies of extreme heat stress days and events during 1971-2014. The increases in human-perceived heat stress are even stronger than air temperature. Urban areas experience more intense heat stress than the surrounding rural areas. We estimate that urbanization accounts for nearly 30% of the increase in mean and extreme heat stress. Urbanization effects are more prominent in the major urban conglomerates such as Beijing-Tianjin-Hebei and the Yangtze and Pearl river deltas. Plain Language Summary Heat stress describes the combined effect of high temperatures and humidity, posing severe impacts on human society and environment, especially in populated urban areas. Here we show that urban areas of eastern China have experienced considerable increases in mean and extreme heat stress during 1971-2014, and these increases are stronger than the surrounding rural areas. Urbanization accelerates the increases in heat stress in urban areas by around 30%. This acceleration is even greater in more populated and developed regions than in less developed regions.