Body dissatisfaction is prevalent worldwide, fueling the highly lucrative diet and plastic surgery industry, with many adverse outcomes. Yet, limited attention has been dedicated to noninvasive interventions that effectively enhance body satisfaction. We argue that, by altering the target of the social comparison process, people can shift their focus from body appearance to physical activity level. The present study investigated whether social comparison in terms of physical activity level provided by fitness apps could affect users' body satisfaction. A survey was conducted with 643 users of WeRun, the Chinese leading mobile fitness app. Subsequent analyses revealed that both upward and downward social comparison mediated the positive relationship between fitness app use and body satisfaction. Moreover, the users' social network size and gender played a moderating role in the social comparison processes. Fitness app use was positively related to downward comparison for male users; social network size moderated the relationship between fitness app use and upward comparison for female users. The utility of fitness apps in mitigating body dissatisfaction is also discussed.