Habitual physical activity is an important component of successful weight loss programs for morbidly obese individuals. This study examined self-reported physical activity (PA) participation in relation to excess weight loss and body mass index (BMI) reduction among gastric bypass surgery patients (GBS). PA participation was hypothesized to contribute to both greater excess weight loss (% EWL) and a greater reduction in BMI at 2 years postsurgery. PA participation was measured via self-report among 1585 GBS patients between 1988 and 2001. GBS patients were assigned to groups [PA (n = 1479)/no PA (n = 106)] and further stratified by presurgical BMI [35–49 kg/m² (n = 897) and 50–70 kg/m² (n = 688)]. Findings showed that GBS patients who reported PA participation were younger [P < 0.0001, PA (40.1 ± 9.9) vs no PA (44.2 ± 11.2)], had greater % EWL [P = 0.0081, PA (68.2 ± 17.4%) vs no PA (63.9 ± 19.5%)], and a greater decrease in BMI [P = 0.0011, PA (18.3 ± 5.7 kg/m²) vs no PA (16.6 ± 5.4 kg/m²)]. When stratified by presurgical BMI, only physically active patients with a BMI of 50–70 kg/m² showed an increase in % EWL [P = 0.0444, PA (63.2 ± 16.5) vs no PA (57.9 ± 17.3)], whereas both BMI groups showed significant reductions in BMI at 2 years [BMI of 35–49 kg/m² P = 0.0184, PA (16.0 ± 4.0 kg/m²) vs no PA (14.4 ± 4.0 kg/m²); and BMI of 50–70 kg/m² P = 0.0221, PA (21.50 ± 6.0 kg/m²) vs no PA (19.7 ± 5.5 kg/m²)], respectively. PA had a favorable effect on % EWL and BMI among GBS patients at 2 years postsurgery, thus supporting the inclusion of habitual PA in a comprehensive GBS postsurgical weight maintenance program.