<p>The mental health impacts of cumulative heat exposure remain poorly understood, particularly regarding the role of social mechanisms. This study investigates whether social isolation mediates the relationship between cumulative heat exposure and mental health among older Chinese adults who have experienced previous heat exposure. Using data from the Chinese Longitudinal Healthy Longevity Survey (2005–2018) and corresponding meteorological records (1974–2019), we quantified cumulative heat exposure as the total number of days during the warm season (May–September) exceeding specific wet-bulb globe temperature thresholds (1-day, 2-day, and 3-day). Social isolation was assessed across four dimensions: core networks, peripheral networks, frequency of family visits, and social participation. Employing ordinary least squares (OLS) regression analyses and bootstrap-based parallel mediation models with clustered standard errors, we found that higher cumulative heat exposure was significantly associated with poorer mental health across all definitions. Mediation analyses further indicated that heat exposure was related to an expansion of core networks and a reduction of peripheral ties, both of which were linked to better mental health outcomes. No mediation effects were observed for family visit frequency or social participation. These findings provide novel evidence that cumulative heat exposure among older adults may trigger an adaptive restructuring of social networks, which in turn could help mitigate adverse mental health impacts.</p>