Insulin icodec is an investigational once-weekly basal insulin analogue for diabetes management.We conducted a 78-week randomized, open-label, treat-to-target phase 3a trial (including a 52-week main phase and a 26-week extension phase, plus a 5-week follow-up period) involving adults with type 2 diabetes (glycated hemoglobin level, 7 to 11%) who had not previously received insulin. Participants were randomly assigned in a 1:1 ratio to receive once-weekly insulin icodec or once-daily insulin glargine U100. The primary end point was the change in the glycated hemoglobin level from baseline to week 52; the confirmatory secondary end point was the percentage of time spent in the glycemic range of 70 to 180 mg per deciliter (3.9 to 10.0 mmol per liter) in weeks 48 to 52. Hypoglycemic episodes (from baseline to weeks 52 and 83) were recorded.Each group included 492 participants. Baseline characteristics were similar in the two groups. The mean reduction in the glycated hemoglobin level at 52 weeks was greater with icodec than with glargine U100 (from 8.50% to 6.93% with icodec [mean change, -1.55 percentage points] and from 8.44% to 7.12% with glargine U100 [mean change, -1.35 percentage points]); the estimated between-group difference (-0.19 percentage points; 95% confidence interval [CI], -0.36 to -0.03) confirmed the noninferiority (P<0.001) and superiority (P = 0.02) of icodec. The percentage of time spent in the glycemic range of 70 to 180 mg per deciliter was significantly higher with icodec than with glargine U100 (71.9% vs. 66.9%; estimated between-group difference, 4.27 percentage points [95% CI, 1.92 to 6.62]; P<0.001), which confirmed superiority. Rates of combined clinically significant or severe hypoglycemia were 0.30 events per person-year of exposure with icodec and 0.16 events per person-year of exposure with glargine U100 at week 52 (estimated rate ratio, 1.64; 95% CI, 0.98 to 2.75) and 0.30 and 0.16 events per person-year of exposure, respectively, at week 83 (estimated rate ratio, 1.63; 95% CI, 1.02 to 2.61). No new safety signals were identified, and incidences of adverse events were similar in the two groups.Glycemic control was significantly better with once-weekly insulin icodec than with once-daily insulin glargine U100. (Funded by Novo Nordisk; ONWARDS 1 ClinicalTrials.gov number, NCT04460885.).