Background: Most patients do not meet the recommended level of physical activity after bariatric surgery, and psychological factors underlying postoperative physical activity remain poorly understood. This study aimed at identifying self-regulatory predictors of physical activity after bariatric surgery. Methods: Questionnaire data including self-regulation variables and the short-version of the International Physical Activity Questionnaire were obtained in a prospective cohort of 230 patients 1 year after Roux-en-Y gastric bypass. The study sample consisted of participants consenting to wear an ActiGraph GT3X+ accelerometer for seven consecutive days, 18–24 months after surgery (n = 120). Results: A total of 112 participants with complete self-report data provided valid accelerometer data. Mean age was 46.8 years (SD = 9.3), and 81.3% was women. Preoperative and postoperative BMI was 44.8 ± 5.5 and 30.6 ± 5.0 kg/m2, respectively. Total weight loss was 28.9% (SD = 7.5). By objective measures, 17.9% of the participants met the recommended level of moderate-to-vigorous-intensity of physical activity of ≥150 min/week, whereas 80.2% met the recommended level according to self-reported measures. Being single, higher education level, and greater self-regulation predicted objective physical activity in multivariate regression analysis. Greater self-regulation also predicted self-reported physical activity. Weight loss 1 year after surgery was not associated with self-reported or objectively measured physical activity. Conclusions: Despite large differences between accelerometer-based and subjective estimates of physical activity, the associations of self-regulatory factors and weight loss with postoperative physical activity did not vary depending on mode of measurement. Self-regulation predicted both objective and self-reported physical activity. Targeting patients’ self-regulatory ability may enhance physical activity after gastric bypass.