The purpose of this study was to develop and validate an ecological model of leisure-time physical activity (LTPA) participation among adults in Tehran, examining behavioral, structural, and contextual determinants. A sequential exploratory mixed-methods design was employed. Initially, semi-structured interviews with experts, policymakers, and citizens were analyzed using grounded theory to identify key determinants, which then resulted in the development of a 125-item questionnaire. This questionnaire was administered to 335 physically active adults, and the data were analyzed using confirmatory factor analysis and structural equation modeling to test the direct and indirect relationships between ecological domains. The findings revealed that behavioral factors, such as psychological constructs, motivation, and self-efficacy, were the main direct predictors of LTPA participation. Contextual and structural factors indirectly influenced engagement by shaping these behavioral pathways. The validated ecological model showed acceptable fit indices, confirming significant relationships between the domains and highlighting the mediating roles of intrapersonal and participatory mechanisms. This research offers a novel, empirically validated ecological model of LTPA participation in an urban Middle Eastern context, integrating qualitative and quantitative findings to provide a comprehensive framework for multi-level interventions that combine behavioral strategies with supportive structural policies and sociocultural initiatives for sustainable physical activity promotion.