Aims: Physical activity (PA) plays an important role in the prevention of cardiovascular disease (CVD), particularly in individuals with type 1 diabetes mellitus (T1DM) who are at increased risk. Our aim was to determine levels of moderate-to-vigorous physical activity (MVPA), sedentary behaviour and sleep in adolescents with T1DM, and identify barriers to PA. Methods: Participants aged 12-18 with T1DM wore an accelerometer and continuous glucose monitor for 24 hours over 7-days. Data was processed into PA metrics and sleep. Pearson correlations were used to test associations between MVPA and metabolic measures. Barriers to PA were measured using a questionnaire. Results: Thirty-seven adolescents provided valid accelerometer data. Mean daily MVPA was 44.0 minutes [SD 17.6] with 16.2% achieving the guideline of ≥ 60 minutes/day. Participants had 11 hours [SD 1.2] of sedentary behaviour and 7.6 hours [SD 1.5] of sleep/day. There was no difference in MVPA in overweight or obese (53.8%) vs. healthy weight (44.2%) adolescents (45.0 minutes [SD 16.6] vs. 43.1 minutes [SD 18.8]). Only 39.6% reported one or more diabetes specific barrier to PA. Conclusion: Adolescents with T1DM engage in insufficient MVPA and sleep, irrespective of body weight status, suggesting the need for targeted interventions.