BACKGROUND: There are currently limited data regarding the effect of semaglutide 2·4 mg in individuals with obesity and prediabetes in clinical trials. We aimed to assess the efficacy and safety of semaglutide 2·4 mg for weight management and glycaemic control in participants with obesity and prediabetes. METHODS: STEP 10 was a randomised, double-blind, parallel-group, phase 3 trial done across 30 trial sites in Canada, Denmark, Finland, Spain, and the UK and included participants aged 18 years or older with a BMI of 30 kg/m or higher and prediabetes according to UK National Institute for Health and Care Excellence criteria (defined as having at least one of the following at screening: HbA of 6·0-6·4% [42-47 mmol/mol] or fasting plasma glucose [FPG] of 5·5-6·9 mmol/L). Participants were randomly assigned (2:1) to once-weekly subcutaneous semaglutide 2·4 mg or placebo with diet and physical activity counselling for 52 weeks, followed by a 28-week off-treatment period. Primary endpoints were percentage change in bodyweight and proportion of participants reverting to normoglycaemia (HbA <6·0% [<42 mmol/mol] and FPG <5·5 mmol/L) at week 52 (assessed in all randomly assigned participants by intention to treat). Selective safety data were collected for participants who received at least one dose of study drug. This trial is registered with ClinicalTrials.gov, NCT05040971, and is complete. FINDINGS: Between Sept 16 and Dec 29, 2021, 138 participants were randomly assigned to semaglutide 2·4 mg and 69 to placebo. 147 (71%) were female and 60 (29%) were male; 183 (88%) were White. All randomly assigned participants received at least one dose of study drug. Baseline mean age was 53 years (SD 11), bodyweight 111·6 kg (22·2), BMI 40·1 kg/m (6·9), waist circumference 120·1 cm (14·7), HbA 5·9% (0·3; 41·3 mmol/mol [3·0]), and FPG 5·9 mmol/L (0·6). There was a significantly greater reduction in bodyweight with semaglutide 2·4 mg than with placebo at week 52 (-13·9% [SD 0·7] vs -2·7% [0·6]; estimated treatment difference -11·2% [95% CI -13·0 to -9·4]; p<0·0001). Greater proportions of participants reverted to normoglycaemia at week 52 with semaglutide 2·4 mg than with placebo (103 [81%] of 127 vs nine [14%] of 64; odds ratio 19·8 [95% CI 8·7 to 45·2]; p<0·0001). Serious adverse events occurred in 12 (9%) participants receiving semaglutide 2·4 mg versus six (9%) receiving placebo. Adverse events leading to treatment discontinuation occurred in eight (6%) participants in the semaglutide 2·4 mg group versus one (1%) participant in the placebo group. No new safety signals were reported. INTERPRETATION: Semaglutide 2·4 mg provided superior reduction in bodyweight and reversion to normoglycaemia versus placebo in participants with obesity and prediabetes. The safety and tolerability profile was consistent with previous studies and with the GLP-1 receptor agonist class. These findings support the potential use of semaglutide 2·4 mg as a treatment option for individuals with obesity and prediabetes to achieve reversion to normoglycaemia. FUNDING: Novo Nordisk. TRANSLATION: For the Spanish translation of the abstract see Supplementary Materials section.