Introduction: Depression is common, increasing among adolescents and carries risk of disability, lower educational achievements, cardiovascular disease, substance abuse, self-harm and suicide. The effects of evidence-based treatments with medication or psychotherapy are modest. Aerobic exercise is a promising intervention for adolescents with depression, but available studies are hampered by methodological shortcomings. This study aims to evaluate aerobic group exercise versus an active comparator of leisure group activities in adolescents from clinical services with mild-to-moderate depression.

Methods and analysis: This study is a multicentre randomised controlled trial at four psychiatric clinics in Sweden. Participants (n=122) will be randomised 1:1 to group exercise delivered by exercise professionals and supported by mental health (MH) workers or leisure activities lead by the same MH workers for 1 hour three times a week for 12 weeks. Participants will be assessed at baseline, single blind after 13 weeks and 26 weeks and openly after 1 year. Participants randomised to the leisure group will be offered exercise in the open phase. The primary outcome is clinician-rated Children's Depression Rating Scale-Revised. Secondary outcomes are self-rated Quick Inventory of Depressive Symptomatology, self-rated functioning; clinician-rated improvement and functioning; objectively measured aerobic capacity, muscular strength, muscular endurance, body composition and presence or activity of selected biological markers of neuroprotection and neuroinflammation in blood samples. Further outcomes are cost-effectiveness and adolescents', parents' and coaches' experiences of the interventions and an exploration of how the adolescents' health and lifestyle are influenced by the interventions through qualitative interviews.

Ethics and dissemination: The study is approved by the Swedish Ethical Review Authority (Ref. 2021-05307-01). Informed consent in writing will be provided from patients and parents of participants below 15 years of age. The results of this study will be communicated to the included participants and healthcare providers and also submitted for publication in peer-reviewed journals. Trial registration number NCT05076214. © 2022 BMJ Publishing Group. All rights reserved.