Importance: Once-weekly insulin icodec could provide a simpler dosing alternative to daily basal insulin in people with type 2 diabetes. Objective: To evaluate the efficacy and safety of once-weekly icodec vs once-daily insulin degludec in people with insulin-naive type 2 diabetes. Design, Setting, and Participants: Randomized, double-masked, noninferiority, treat-to-target, phase 3a trial conducted from March 2021 to June 2022 at 92 sites in 11 countries in adults with type 2 diabetes treated with any noninsulin glucose-lowering agents with hemoglobin A_1c(HbA_1c) of 7%-11% (53-97 mmol/mol). Interventions: Participants were randomly assigned in a 1:1 ratio to receive either once-weekly icodec and once-daily placebo (icodec group; n = 294) or once-daily degludec and once-weekly placebo (degludec group; n = 294). Main Outcomes and Measures: The primary end point was change in HbA_1cfrom baseline to week 26 (noninferiority margin, 0.3% percentage points). Secondary end points included change in fasting plasma glucose from baseline to week 26, mean weekly insulin dose during the last 2 weeks of treatment, body weight change from baseline to week 26, and number of level 2 (clinically significant; glucose level <54 mg/dL) and level 3 (severe; requiring external assistance for recovery) hypoglycemic episodes. Results: Among 588 randomized participants (mean [SD] age, 58 [10] years; 219 [37%] women), 564 (96%) completed the trial. Mean HbA_1clevel decreased from 8.6% (observed) to 7.0% (estimated) at 26 weeks in the icodec group and from 8.5% (observed) to 7.2% (estimated) in the degludec group (estimated treatment difference [ETD], -0.2 [95% CI, -0.3 to -0.1] percentage points), confirming noninferiority (P <.001) and superiority (P =.002). There were no significant differences between the icodec and degludec groups for fasting plasma glucose change from baseline to week 26 (ETD, 0 [95% CI, -6 to 5] mg/dL; P =.90), mean weekly insulin dose during the last 2 weeks of treatment, or body weight change from baseline to week 26 (2.8 kg vs 2.3 kg; ETD, 0.46 [95% CI, -0.19 to 1.10] kg; P =.17). Combined level 2 or 3 hypoglycemia rates were numerically higher in the icodec group than the degludec group from week 0 to 31 (0.31 vs 0.15 events per patient-year exposure; P =.11) and statistically higher in the icodec group from week 0 to 26 (0.35 vs 0.12 events per patient-year exposure; P =.01). Conclusions and Relevance: Among people with insulin-naive type 2 diabetes, once-weekly icodec demonstrated superior HbA_1creduction to once-daily degludec after 26 weeks of treatment, with no difference in weight change and a higher rate of combined level 2 or 3 hypoglycemic events in the context of less than 1 event per patient-year exposure in both groups. Trial Registration: ClinicalTrials.gov Identifier: NCT04795531.