Numerous studies have demonstrated that urban development exacerbated the urban heat island effect. With the implementation of new urbanization strategies, urban agglomerations have gradually become the predominant form of China’s urbanization process. Does the development of urban agglomerations cause the change of regional thermal environment and intensify the urban heat island effect? How does the development of urban agglomeration affect the evolution of regional heat island spatial pattern? These questions are crucial for the sustainable development of urban agglomeration and urgently require scientific answers. This study focused on four urban agglomerations in China: Beijing-Tianjin-Hebei (BTH) urban agglomeration, Yangtze River Delta (YRD) urban agglomeration, Changsha-Zhuzhou-Xiangtan (CZT) urban agglomeration, and Pearl River Delta (PRD) urban agglomeration. Using the land surface temperature data from the summer months of 2000 to 2020, we first identified the regional heat islands within these urban agglomerations, then explored the spatial pattern characteristics and their changes over time, and finally analyzed the quantitative relationship between the patterns and the proportion of impervious surface in the four urban agglomerations. The results revealed that: (1) From 2000 to 2020, the scale of these urban agglomerations increased rapidly, leading to a significant rise in the area of regional heat islands. (2) There was significant change in the spatial pattern of regional heat islands, with the BTH urban agglomeration and the PRD urban agglomeration showing agglomeration characteristics, as evidenced by an increase in the average size of heat island patches and a decrease in their average nearest distance. In contrast, the YRD urban agglomeration and CZT urban agglomeration showed an increase in both the average size of heat island patches and their average nearest distance. (3) With the development of urban agglomerations, the regional heat island patch connectivity levels of the four urban agglomerations increased significantly. However, the trends of temperature similarity level among urban areas in the four urban agglomerations were different. Among them, with the development of YRD urban agglomeration and CZT urban agglomeration, the temperature similarity among cities increased rapidly at first. When the proportion of impervious surface in urban agglomerations reached 20% and 4%, respectively, the temperature similarity among the urban areas was high and tended to be stable. The research can enhance the understanding on how urban agglomeration development affects the regional thermal environment, and thereby provide scientific evidence for the coordinated management and mitigation strategies to address urban agglomeration thermal environment issues for sustainable regional development.