Bariatric surgery (or weight loss surgery) includes a variety of procedures performed on people who are obese. Long term weight loss through the standard of care procedures (Roux en-Y bypass, sleeve gastrectomy, and biliopancreatic diversion with duodenal switch) is largely achieved by altering gut hormone levels responsible for hunger and satiety, leading to a new hormonal weight set point. Bariatric surgery is the most effective treatment causing weight loss and reducing complications of obesity.

As of October 2022, the American Society of Metabolic and Bariatric Surgery (ASMBS) and International Federation for the Surgery of Obesity (IFSO) recommend bariatric surgery for adults with a body mass index (BMI) >35, regardless of obesity-associated conditions, and recommend considering surgery for people with BMI 30.0-34.9 who have metabolic disease. This is a recent change in guidelines (October 2022), so other guideline-producing organizations and health insurance plans may take time before their guidelines are updated.

Bariatric surgery can have significant health benefits in addition to weight loss, including improvement in cardiovascular risk factors, fatty liver disease, diabetes management, and reduction in mortality. Long-term studies from 2009 show the procedures result in significant long-term loss of weight, recovery from diabetes, improvement in cardiovascular risk factors, and a mortality reduction from 40% to 23%. A meta-analysis in 2021 found that bariatric surgery was associated with 59% and 30% reduction in all-cause mortality among obese adults with or without type 2 diabetes, respectively. This meta-analysis also found that median life-expectancy was 9.3 years longer for obese adults with diabetes who received bariatric surgery as compared to routine (non-surgical) care, whereas the life expectancy gain was 5.1 years longer for obese adults without diabetes. A 2013 National Institute of Health symposium summarizing available evidence found a 29% mortality reduction, a 10-year remission rate of type 2 diabetes of 36%, fewer cardiovascular events, and a lower rate of diabetes-related complications in a long-term, non-randomized, matched intervention 15–20 year follow-up study, the Swedish Obese Subjects Study. The symposium also found similar results from a Utah study using more modern gastric bypass techniques, though the follow-up periods of the Utah studies are only up to seven years. While randomized controlled trials of bariatric surgery exist, they are limited by short follow-up periods. The risk of death in the period following surgery is less than 1 in 1,000.