Abstarct: The purpose of this research was to investigate the effect of social responsibility of football clubs on the citizenship behavior of fans with the mediating role of social capital. In terms of the purpose, the present research is of the type of applied research; In terms of the implementation method, it is descxriptive-correlation research, and in terms of data collection, it is non-experimental and survey research, which is done by modeling method baxsed on structural equation model. The statistical population of the present research includes all the fans of the selected clubs (Estiqlal, Persepolis, Sepahan and others) in the Premier Football League of Iran, whose number is uncertain. Among them, 160 people were selected as the research sample using G-Power software, using available sampling method. To collect data, the demographic characteristics sheet, Carroll's (1999) social responsibility questionnaire, Yi and Gong's (2013) citizenship behavior questionnaire, and Nahapit and Ghoshal's (1998) social capital questionnaire were used. Structural equation modeling method has been used to check the research hypotheses. The results showed that social responsibility had a significant effect on social capital. Social responsibility had a significant impact on fans' citizenship behavior. Social capital had a significant effect on fans' citizenship behavior. Also, social capital plays a mediating role in relation to social responsibility and citizenship behavior of fans. The results obtained from the research show that increasing social capital as a mediator, It plays an important role in promoting the citizenship behavior of fans by strengthening the social responsibility of football clubs. This communication is beneficial for both parties, i.e. clubs and fans.