The relationship of physical activity with weight loss may largely be due to its association with psychosocial factors. The goal of this research was to clarify such relationships using a field design lasting 24 weeks. In Study 1, change in self-regulation for controlled eating, but not energy expenditure, mediated the relationship between changes in physical activity and weight in formerly sedentary, severely obese adults (n = 174). In Study 2 (n = 148), the addition of a cognitive-behavioral nutrition treatment was associated with significantly greater improvement in self-regulation for eating. Physical activity-related self-regulation changes were related to those improvements. Changes in self-efficacy for controlled eating and mood mediated the prediction of changes in eating-related self-regulation from changes on physical activity-related self-regulation. Change in body satisfaction was not a significant mediator. Based on the findings, practical uses of physical activity to enhance self-regulatory skills for controlled eating were suggested.