Background: Physical activity is a strong predictor of sustaining weight loss. Yet physical activity has been challenging to maintain. Adolescent bariatric surgery is increasing, and there is typically an initial 6-month period when improving health behaviors such as physical activity are addressed by a clinic-based team. However, there is minimal understanding of how to target psychosocial factors relevant for behavioral changes. Methods: A group of 15 adolescent candidates for bariatric surgery (mean age = 15.1 y; mean body mass index = 55.9 kg/m2) were assessed on changes in 3 theory-based predictors of physical activity from baseline-month 3 and baseline-month 6. Results: Changes in physical activity-related self-regulation and self-efficacy over 3 months significantly predicted change in physical activity over 6 months. Reciprocal relationships were also significant, including the prediction of physical activity change by change in negative mood. The clinical psychology-based factor of weight-related quality-of-life significantly moderated the prediction of self-regulation via physical activity, and degree of depressive symptoms significantly moderated the prediction of changes in physical activity through self-efficacy changes. Conclusions: Because improvements in several theory-based psychosocial variables related to physical activity have demonstrated a carry-over to controlling eating, the improved understanding of those variables for treating adolescents with severe obesity was useful.