The rapid development of urban megaregions has changed the regional thermal environment by raising the land surface temperature (LST) in a continuous area. The thermal environment problem in urban megaregions has become a research focus. This study aimed to investigate the relationship between the LST and the external expansion and internal spatial patterns of impervious surfaces area (ISA) in Guangdong – Hong Kong – Macao Greater Bay Area (GBA). The Landsat and MODIS data, as well as various geospatial approaches, including buffer-based zoning, impervious surface density gradients and landscape-based metrics statistics, were used to facilitate the analysis. We found that the areas of ISA increased from 1697.293 km² in 2000 to 9754.936 km² in 2015 in the GBA, expanding to the east, southeast and south. The mean LST increased by 7.41 °C and 10.23 °C in the daytime and nighttime, respectively. The internal density of ISA had increased significantly in the past 15 years. The highest ISA density and the highest LST appeared in the 15 km buffer zone closest to study area's center. And the LST was decreased by 0.15 °C for each 15 km increased in the distance to the study area's center. There was a significant strong positively correlation between the LST and the density of the ISA. The LST of the very-high-density area of the ISA was about 2.85 °C higher than that of the very-low-density area. The internal landscape pattern of ISA was strongly correlated with the LST. The study highlight that the center of urban megaregion had the larger ISA patches, which were simpler in shape, more aggregated and connected, and less fragmented seemed to exacerbate elevate in the LST in successive regions. In particular, the study found that the nighttime LST were affected by the environmental characteristics of urban megaregion. The correlation between the LST and the ISA was higher at daytime than at nighttime.