Background: Data on the benefits of the once weekly GLP-1 receptor agonist semaglutide 2·4 mg for weight management in people from east Asia are insufficient. The objective of this study was to determine the efficacy and safety of once weekly semaglutide 2·4 mg versus placebo for weight management in a predominantly east Asian adult population., Methods: This randomised phase 3a, double-blind multicentre controlled trial (STEP 7) recruited participants from 23 hospitals and trial centres in China, Hong Kong, Brazil, and South Korea. Adults with overweight or obesity, with or without type 2 diabetes, were randomly assigned (2:1) to receive a subcutaneous injection of either semaglutide 2·4 mg or placebo once a week for 44 weeks, plus a diet and physical activity intervention. Randomisation was done in blocks of six with an interactive web response system and was stratified by diagnosis of type 2 diabetes. Participants, investigators, and the trial sponsor were masked to treatment allocation until after database lock. Primary endpoints were percentage change in mean bodyweight and proportion of participants having reached a weight reduction of at least 5% of bodyweight from baseline to week 44. Safety was assessed in all participants who received at least one dose of study drug. This study is registered with ClinicalTrials.gov, NCT04251156, and is now complete., Findings: From Dec 8, 2020, to Aug 23, 2022, 448 participants were screened, of whom 375 were randomly assigned to either the semaglutide 2·4 mg group (n=249) or the placebo group (n=126). Estimated mean percentage change in bodyweight from baseline to week 44 was -12·1% (SE 0·5) with semaglutide 2·4 mg versus -3·6% (0·7) with placebo (estimated treatment difference -8·5 percentage points [95% CI -10·2 to -6·8]; p<0·0001). At week 44, the proportion of participants who lost 5% or more of their bodyweight was higher in the semaglutide 2·4 mg group than in the placebo group (203/238 [85%] vs 36/116 [31%]); odds ratio 13·1 (95% CI 7·4-23·1; p<0·0001). Adverse events were reported by 231 (93%) of 249 participants in the semaglutide 2·4 mg group and 108 (86%) of 126 participants in the placebo group, the most common of which were gastrointestinal disorders (168/249, 67% vs 45/126, 36%)., Interpretation: The results of this study support the use of semaglutide 2·4 mg for weight management in people of east Asian ethnicity with overweight or obesity and with or without type 2 diabetes., Funding: Novo Nordisk., Translations: For the Mandarin, Portuguese and South Korean translations of the abstract see Supplementary Materials section., Competing Interests: Declaration of interests YM reports having received honoraria and personal fees from Eli Lilly Diabetes, Novo Nordisk, and Sanofi outside the submitted work. XB is a full-time employee of Novo Nordisk (China) Pharmaceuticals, Beijing, China, and is a shareholder in Novo Nordisk. FGE has received clinical research grants from Novo Nordisk, Eli Lilly, AstraZeneca, Novartis, Eurofarma, and Kowa; is on advisory boards for Novo Nordisk, Eli Lilly, AstraZeneca, and Pfizer; and has given lectures for AstraZeneca, Bayer, Novo Nordisk, Eli Lilly, Sanofi, Abbot, Merck, and Novartis. MRH is a full-time employee of Novo Nordisk and shareholder in Novo Nordisk and Eli Lilly; before his current employment MRH received speaking fees from Novartis outside the submitted work. BTK has received research funding from Boehringer Ingelheim, Eli Lilly, and Novo Nordisk for carrying out clinical trials or studies; speaker honoraria from Alvogen Korea and Novo Nordisk; all proceeds have been donated to the Ajou University Hospital to support obesity research. AK is an employee of and shareholder in Novo Nordisk. RCWM has received research funding from AstraZeneca, Bayer, Novo Nordisk, Pfizer, Roche Diagnostics Hong Kong and Tricida for carrying out clinical trials, and has received speaker honorarium or consultancy in advisory boards from AstraZeneca, Bayer, Boehringer Ingelheim, Medtronic, Merck, and Kyowa Kirin; all proceeds have been donated to the Chinese University of Hong Kong to support diabetes research. NZ is an employee of and shareholder in Novo Nordisk. ML has received consulting fees from Novo Nordisk and Sanofi, and honoraria from Novo Nordisk, Sanofi, and Bayer, and is a committee member of the Chinese Society of Endocrinology. TY declares no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved.)