Background: People with serious mental illness have high rates of obesity and related medical problems, and die years prematurely, most commonly from cardiovascular disease. Specialized, in-person weight management interventions result in weight loss in efficacy trials with highly motivated patients. In usual care, patient enrollment and retention are low with these interventions, and effectiveness has been inconsistent. Objective: To determine whether computerized provision of weight management with peer coaching is feasible to deliver, is acceptable to patients, and is more effective than in-person delivery or usual care. Design: Mixed-methods randomized controlled trial. Participants: Two hundred seventy-six overweight patients with serious mental illness receiving care at a Veterans Administration medical center. Interventions: Patients were randomized to 1) computerized weight management with peer coaching (WebMOVE), 2) in-person clinician-led weight services, or 3) usual care. Both active interventions offered the same educational content. Main Measures: Body mass index; and feasibility and acceptability of the intervention. Key Results: At 6 months, in obese patients (n = 200), there was a significant condition by visit effect (F = 4.02, p = 0.02). The WebMOVE group had an average estimated BMI change from baseline to 6 months of 34.9 ± 0.4 to 34.1 ± 0.4. This corresponds to 2.8 kg (6.2 lbs) weight loss (t = 3.2, p = 0.001). No significant change in BMI was seen with either in-person services (t = 0.10, p = 0.92), or usual care (t = −0.25, p = 0.80). The average percentage of modules completed in the WebMOVE group was 49% and in the in-person group was 41% (t = 1.4, p = 0.17). When non-obese patients were included in the analyses, there was a trend towards a condition by visit effect (F = 2.8, p = 0.06). WebMOVE was well received, while the acceptability of in-person services was mixed. Conclusions: Computerized weight management with peer support results in lower weight, and can have greater effectiveness than clinician-led in-person services. This intervention is well received, and could be feasible to disseminate.