We evaluated a cognitive behaviour therapy–based programme to improve glycaemic control and psychosocial wellbeing in adolescents with type 1 diabetes. A total of 147 adolescents aged 13–16 years were randomized to the intervention (n = 73) or standard care (n = 74). The primary outcome was glycaemic control at 3 and 12 months post randomization, and secondary measures were stress, self-efficacy and quality of life. Mixed-effects regression models were used to assess differences in means between groups at each time point. There was little evidence of differences in glycaemic control between groups. However, psychosocial wellbeing improved in the intervention group compared to the control group. Recommendations for future programmes are discussed. The trial is registered with the Australian New Zealand Clinical Trials Registry (ACTRN12608000368336).