BackgroundPromoting habitual physical activity (PA) among bariatric patients is crucial for long-term surgery success, yet it poses a significant challenge for healthcare practitioners. ObjectiveThis randomized controlled trial (RCT) aimed to examine the effectiveness of a theory-based behavioral intervention on PA level in post-metabolic bariatric surgery (MBS) patients. SettingUniversity Hospital, Israel MethodsForty-four patients undergoing MBS were randomized to the intervention (n=22) or control group (n=22). The intervention group received a six-month PA counseling program based on self-determination theory (SDT) and the five A’s framework, while the control group received usual care. PA level and exercise self-efficacy (SEE) were assessed pre- (baseline, pre-intervention) and post-operatively (14- and 28-weeks follow-ups). Anthropometrics, physical function, cardiovascular, and biochemical outcomes were measured at all time points. ResultsThe intervention group showed greater increases in PA levels across time compared to the control group. A significant increase in step counts from baseline to 14 weeks was observed (p=0.003). Significant differences favoring the intervention group were observed in self-reported PA changes from baseline to 14- and 28-week follow-ups (p=0.020 and p=0.024, respectively). Additionally, Physical function, as assessed by the 6-minute walk (6MWT) and the five sit-to-stand (5STS) test, significantly improved in the intervention group throughout the follow-up period (p<0.05 for all), However, no between-group differences were observed in SEE, anthropometrics, cardiovascular or biochemical parameters. ConclusionA six-month behavioral intervention post-MBS significantly enhanced patients' PA levels and physical function. Given its theory-based approach and structured protocol, this intervention could be disseminated to support MBS clinicians and centers. Further research with longer follow-up period is warranted to confirm these findings and assess the long-term effects.