BACKGROUND: Continuous glucose monitoring (CGM) can provide a comprehensive assessment of glycaemic control. This exploratory analysis of the ONWARDS 1 trial assessed CGM-based metrics and CGM-derived hypoglycaemia duration in insulin-naive individuals with type 2 diabetes treated with subcutaneous once-weekly insulin icodec (icodec) versus once-daily insulin glargine U100 (glargine U100). METHODS: ONWARDS 1 was a 78-week (52-week main treatment phase and a 26-week treatment extension phase plus a 5-week follow-up), randomised, open-label, treat-to-target, phase 3a trial done at 143 sites (outpatient clinics and hospital departments) across 12 countries. Adults (aged ≥ 18 years) with type 2 diabetes (HbA(1c): 7·0-11·0%) who had not previously received insulin were randomly assigned (1:1) via an interactive web-response system to once-weekly icodec or once-daily glargine U100. Double-masked CGM data were collected during treatment initiation (weeks 0-4), midtrial (weeks 22-26), end of main phase (weeks 48-52), end of extension phase (weeks 74-78), and follow-up (weeks 78-83). Secondary and exploratory outcomes were CGM-based metrics, including the mean percentages of time in glycaemic range (TIR; sensor glucose 3·9-10·0 mmol/L [70-180 mg/dL]), time in tight range (TITR; 3·9-7·8 mmol/L [70-140 mg/dL]), time above range (TAR; >10·0 mmol/L [>180 mg/dL]), and time below range (TBR; <3·9 mmol/L [<70 mg/dL] and <3·0 mmol/L [<54 mg/dL]), and CGM-derived hypoglycaemic episode durations (episodes defined by sensor glucose <3·9 mmol/L [<70 mg/dL for ≥ 15 consecutive minutes]). Analyses were done in the full analysis set (all randomly assigned participants). The ONWARDS 1 trial is registered with ClinicalTrials.gov, NCT04460885, and is complete. FINDINGS: Participants were enrolled and randomly assigned in ONWARDS 1 between Nov 25, 2020, and Dec 1, 2022 (n=492 in each treatment group). During treatment initiation, we observed no statistically significant differences in the mean percentages of TIR, TITR, TAR, and TBR with icodec versus glargine U100. During the midtrial, end of main phase, and end of extension phase periods, the mean percentages of TIR and TITR were statistically significantly greater and the mean percentages of TAR statistically significantly lower with icodec versus glargine U100. The mean percentages of TIR met the internationally recommended CGM target (>70%) with icodec but not with glargine U100 during the three periods. TBR (<3·9 mmol/L [<70 mg/dL] and <3·0 mmol/L [<54 mg/dL]) was low and below recommended targets (<4% and <1%, respectively) across all study periods in both treatment groups, with no statistically significant differences between treatment groups for the lower threshold (<3·0 mmol/L [<54 mg/dL]). During the follow-up period, mean percentages of TIR, TITR, TAR, and TBR did not statistically significantly differ with icodec versus glargine U100. The duration of overall hypoglycaemic episodes was similar between treatment groups throughout the trial (median duration ≤ 35 min). INTERPRETATION: These CGM data support the long-term efficacy and safety of icodec versus glargine U100 during treatment and indicated no increase in the duration of individual hypoglycaemic episodes with icodec versus glargine U100 in insulin-naive individuals with type 2 diabetes. FUNDING: Novo Nordisk.