Based on an extended Theory of Planned Behavior (ETPB), this study investigated how perceived social sustainability shaped individuals’ intentions to use fitness facilities. Specifically, it examined the moderating role of social sustainability in the relationships between key TPB determinants and fitness facility use intention. Survey data were collected from 195 adults living in metropolitan areas and were analyzed using partial least squares structural equation modeling (PLS-SEM). The results revealed that perceived social sustainability exerted a dual moderating influence on intention formation by strengthening the effect of subjective norm while attenuating the effect of perceived behavioral control on use intention. Higher levels of perceived social sustainability (SS) strengthened the relationship between subjective norm (SN) and fitness facility use intention with a medium effect size, while attenuating the relationship between perceived behavioral control (PBC) and use intention with a small effect size. In contrast, no significant moderating effect was observed in the relationship between attitude and use intention. These findings suggest that value-oriented considerations related to social responsibility and community well-being enhanced socially driven motivations while reducing the relative influence of control-based factors. By demonstrating the conditional effects of perceived social sustainability within the TPB framework, this study extended existing research on health-related behavioral intentions. The findings further highlight the importance of incorporating community-oriented and socially responsible practices into fitness facility management to foster sustainable user engagement.