Obesity is a chronic, relapsing disease associated with multiple complications and a substantial morbidity, mortality and health care burden. Pharmacological treatments for obesity provide a valuable adjunct to lifestyle intervention, which often achieves only limited weight loss that is difficult to maintain. The Semaglutide Treatment Effect in People with obesity (STEP) clinical trial programme is evaluating once-weekly subcutaneous semaglutide 2.4 mg (a glucagon-like peptide-1 analogue) in people with overweight or obesity. Across STEP 1, 3, 4 and 8, semaglutide 2.4 mg was associated with mean weight losses of 14.9%-17.4% in individuals with overweight or obesity without type 2 diabetes from baseline to week 68; 69%-79% of participants achieved ≥10% weight loss with semaglutide 2.4 mg (vs. 12%-27% with placebo) and 51%-64% achieved ≥15% weight loss (vs. 5%-13% with placebo). In STEP 5, mean weight loss was −15.2% with semaglutide 2.4 mg versus −2.6% with placebo from baseline to week 104. In STEP 2 (individuals with overweight or obesity, and type 2 diabetes), mean weight loss was −9.6% with semaglutide 2.4 mg versus −3.4% with placebo from baseline to week 68. Improvements in cardiometabolic risk factors, including high blood pressure, atherogenic lipids and benefits on physical function and quality of life were seen with semaglutide 2.4 mg. The safety profile of semaglutide 2.4 mg was consistent across trials, primarily gastrointestinal adverse events. The magnitude of weight loss reported in the STEP trials offers the potential for clinically relevant improvement for individuals with obesity-related diseases.