This study aimed to develop and test a structural model examining the role of culture and the urban environment in sport participation motivation and the mental health of citizens, with a specific focus on the metropolis of Tehran. Adopting a quantitative, correlational research design, data were collected through field surveys. The statistical population consisted of Tehran residents; using stratified random sampling across the city’s 22 districts, 380 valid questionnaires were analyzed. Data collection instruments included the Goldberg and Hiller General Health Questionnaire (GHQ-28), the Sport Motivation Scale (SMS), the Neighborhood Environment Walkability Scale (NEWS), and a researcher-developed urban culture questionnaire. Data were analyzed using variance-based structural equation modeling (PLS-SEM) via SmartPLS 4 software. The findings indicated that urban culture and the urban environment have direct and significant effects on sport participation motivation. Furthermore, culture, the urban environment, and sport motivation significantly influenced mental health. Given the scoring method of the General Health Questionnaire (where higher scores indicate poorer health), the negative path coefficients leading to mental health represent a reduction in psychological distress and an improvement in mental health. The coefficients of determination (R²) were 0.297 for sport motivation and 0.414 for mental health, indicating the model’s acceptable explanatory power. The results also confirmed the mediating role of sport participation motivation in the relationship between culture/urban environment and mental health. Based on these findings, physical environmental quality, safety, aesthetics, accessibility, and the manifestation of Iranian-Islamic values in urban spaces can reduce anxiety and depression, and promote the mental well-being of citizens by enhancing sport participation motivation.