This study explored the relationship between implicit theories of the body (ITB) and self-reported physical activity. ITB are beliefs about the malleability of the body. Individuals may hold entity ITB (that body appearance is fixed) or incremental ITB (that body appearance can be changed). Female undergraduate students (N = 313) completed an online survey assessing ITB, physical activity, and discrepancy between actual and ideal body weight. Participants endorsing incremental ITB reported more physical activity. A significant ITB by weight discrepancy interaction emerged. Developing interventions targeting implicit theories of the body may be one way to increase physical activity.