AIM: To evaluate the safety and efficacy of once-weekly subcutaneous semaglutide as monotherapy or combined with an oral antidiabetic drug (OAD) vs an additional OAD added to background therapy in Japanese people with type 2 diabetes (T2D) inadequately controlled on diet/exercise or OAD monotherapy. METHODS: In this phase III, open-label trial, adults with T2D were randomized 2:2:1 to semaglutide 0.5 mg or 1.0 mg, or one additional OAD (a dipeptidyl peptidase-4 inhibitor, biguanide, sulphonylurea, glinide, α-glucosidase inhibitor or thiazolidinedione) with a different mode of action from that of background therapy. The primary endpoint was number of adverse events (AEs) after 56 weeks. RESULTS: Baseline characteristics were balanced between treatment arms (601 randomized). More AEs were reported in the semaglutide 0.5 mg (86.2%) and 1.0 mg (88.0%) groups than in the additional OAD group (71.7%). These were typically mild/moderate. Gastrointestinal AEs were most frequent with semaglutide, which diminished over time. The mean glycated haemoglobin (HbA1c) concentration (baseline 8.1%) was significantly reduced with semaglutide 0.5 mg and 1.0 mg vs additional OAD (1.7% and 2.0% vs 0.7%, respectively; estimated treatment difference [ETD] vs additional OAD -1.08% and -1.37%, both P < .0001). Body weight (baseline 71.5 kg) was reduced by 1.4 kg and 3.2 kg with semaglutide 0.5 mg and 1.0 mg, vs a 0.4-kg increase with additional OAD (ETD -1.84 kg and -3.59 kg; both P < .0001). For semaglutide-treated participants, >80% achieved an HbA1c concentration <7.0% (Japanese Diabetes Society target). CONCLUSIONS: Semaglutide was well tolerated, with no new safety issues identified. Semaglutide treatment significantly reduced HbA1c and body weight vs additional OAD treatment in Japanese people with T2D.