Improving our understanding of the spatial pattern of the urban heat island (UHI) effect is an important aspect of current studies on the urban thermal environment. In this study, we applied a UHI footprint (UHI FP) analysis for the metropolitan region of Beijing using a two-dimensional Gaussian model, based on the Landsat series and Moderate Resolution Imaging Spectroradiometer (MODIS) daytime datasets. The temporal trend of the summer UHI FP metrics and their possible associations with urbanization factors (i.e., GDP, population, and built-up area in statistical yearbook) were explored. We found that the study area experienced an extensive UHI effect in the daytime during summer, with more than 2 °C UHI intensity and a footprint of over 1900 km². During the period 2000–2019, the spatial extent of the UHI FP exhibited a significant increasing trend, including nearly 20% increase in the UHI FP region with higher thermal risk and nearly 30% in the ordinary UHI FP region. Moreover, the UHI FP showed a significant association with urbanization factors, indicating that economic, population, and urban land uses are responsible for the expansion of the UHI effect. This study highlights the importance of footprint analysis to depict the dynamic characteristics of the thermal environment of cities. Further UHI FP study is required to provide comprehensive references for sustainable urban planning and development.