ImportanceLaparoscopic sleeve gastrectomy (SG) and laparoscopic Roux-en-Y gastric bypass (RYGB) are widely used bariatric procedures for which comparative efficacy and safety remain unclear.ObjectiveTo compare perioperative outcomes in SG and RYGB.Design, Setting, and ParticipantsIn this registry-based, multicenter randomized clinical trial (Bypass Equipoise Sleeve Trial), baseline and perioperative data for patients undergoing bariatric surgery from October 6, 2015, to March 31, 2022, were analyzed. Patients were from university, regional, county, and private hospitals in Sweden (n = 20) and Norway (n = 3). Adults (aged ≥18 years) eligible for bariatric surgery with body mass indexes (BMIs; calculated as weight in kilograms divided by height in meters squared) of 35 to 50 were studied.InterventionsLaparoscopic SG or RYGB.Main Outcomes and MeasuresPerioperative complications were analyzed as all adverse events and serious adverse events (Clavien-Dindo grade >IIIb). Ninety-day mortality was also assessed.ResultsA total of 1735 of 14 182 eligible patients (12%; 1282 [73.9%] female; mean (SD) age, 42.9 [11.1] years; mean [SD] BMI, 40.8 [3.7]) were included in the study. Patients were randomized and underwent SG (n = 878) or RYGB (n = 857). The mean (SD) operating time was shorter in those undergoing SG vs RYGB (47 [18] vs 68 [25] minutes; P < .001). The median (IQR) postoperative hospital stay was 1 (1-1) day in both groups. The 30-day readmission rate was 3.1% after SG and 4.0% after RYGB (P = .33). There was no 90-day mortality. The 30-day incidence of any adverse event was 40 (4.6%) and 54 (6.3%) in the SG and RYGB groups, respectively (odds ratio, 0.71; 95% CI, 0.47-1.08; P = .11). Corresponding figures for serious adverse events were 15 (1.7%) for the SG group and 23 (2.7%) for the RYGB group (odds ratio, 0.63; 95% CI, 0.33-1.22; P = .19).Conclusions and RelevanceThis randomized clinical trial of 1735 patients undergoing primary bariatric surgery found that both SG and RYGB were performed with a low perioperative risk without clinically significant differences between groups.Trial RegistrationClinicalTrials.gov Identifier: NCT02767505