Background: In recent years, social capital, and especially its importance among the elderly has attracted the attention of researchers. Moreover, it was suggested that older adults lack sufficient physical activity that may be due to the lack of motivation. Accordingly, this study was conducted to determine the relationship between social capital and physical activity participation motivation among the elderly living in the west of Tehran City, Iran, in 2020. Methods: This was a descriptive correlational study. The research population consisted of aged individuals present in the public areas of the west of Tehran City, Iran (districts 5, 9, 21, & 22 of Tehran Municipality), such as sidewalks and passages, shopping centers, parks, mosques, and so on. In total, 400 subjects were recruited by the multistage sampling method. The Abbreviated Mental Test (AMT), Participation Motivation Questionnaire for Older Adults (PMQOA), and Onyx and Bullen social capital scale were used to gather the required data. The obtained data were analyzed by Pearson’s correlation coefficient, Independent Samples t-test, and Analysis of Variance (ANOVA) in SPSS at the significance level of P<0.05. Results: The Mean±SD social capital of the study subjects was equal to 86.27±23.3. The fitness and social factors of activity participation motives had the highest (49.72±19.34) and lowest (18.97±15.96) Mean±SD values (based on an index of 0-100). Social capital was significantly associated with physical activity participation motivators (except fitness & medical subscales) (P<0.05). Social capital was also associated with occupational status, the number of children, house size, and the economic status of the research subjects (P<0.05). Conclusion: According to the obtained results, the social capital of the elderly participating in this study was pretty low. Furthermore, there was a significant relationship between social capital and the motivators of physical activity participation (apart from fitness & medical); this result signifies the necessity of attention to the social capital of the elderly.