Social enterprises have a vital role in improving individuals’ social and economic well-being. In this study we examined the impact of organizational legitimacy on customer citizenship behavior (CCB) within social enterprises, focusing on the mediating mechanism of social network relationship strength, and the moderating effect of network heterogeneity. We proposed and tested our theoretical model using a survey of 381 individual customers who had engaged with social enterprises in Mainland China. The results showed that network relationship strength partially mediated the impact of organizational legitimacy on CCB, and network heterogeneity negatively moderated the effect of organizational legitimacy on CCB but positively moderated the effect of network relationship strength on CCB. Our findings extend understanding of how CCB arises in social enterprises and enrich the social enterprise and CCB literature by adding new empirical evidence from China.