<p>Urban space is a dynamic domain, and areas once designated as greenbelts are often released to secure land for development. Deregulation of greenbelts may alter the urban thermal environment and increase spatial heat risk. This study examined changes in the urban thermal environment following greenbelt deregulation using a Heat Risk Index (HRI). The analysis was conducted in Seoul, South Korea, including a southern district where a greenbelt had been removed. Heat risk was assessed spatially using GIS based on repeated HRI calculations with publicly available data. The HRI was calculated by integrating hazards, exposure, and vulnerability. A historical before-and-after comparison was conducted to evaluate changes in heat risk following greenbelt deregulation, and a high-resolution HRI map was produced for the entire Seoul. The spatial distribution of heat risk in Seoul was influenced not only by average temperature and population density, but also by social vulnerability and inequality in response infrastructure. The analysis of the southern district demonstrated a significant increase in HRI following greenbelt deregulation, driven by reduced green space and increased population density. These findings underscore the need for multidimensional, spatially integrated assessments when formulating urban adaptation strategies in response to future environmental change.</p>