BACKGROUND: Depression is the leading cause of disability in young people (aged 15-25) globally. Loneliness is a major factor in the development and relapse of depression in young people, yet few interventions directly address loneliness. Groups 4 Health (G4H) - a novel, theoretically derived group psychotherapy intervention - may address this disconnect. Previous trials (Phase I and Phase II) have found G4H to be efficacious in reducing symptoms of depression. However, the efficacy of G4H compared to current evidence-based treatments (Phase III) has not been investigated. This protocol details the design and methodology of the Connecting Adolescents to Reduce Relapse (CARR) trial, a randomised control trial assessing the efficacy of G4H in young people relative to cognitive behavioural therapy (CBT). METHODS: The CARR trial is a two-arm non-inferiority randomised controlled trial that will compare the efficacy of G4H to the most widely used evidence-based treatment for depression, CBT, at program completion and 6- and 12-month follow up. Participants will be 200 young people (aged 15-25) with symptoms of depression and/or loneliness recruited from community and university mental health services. We hypothesise that the interventions will be comparable in reducing depression symptoms, but that G4H will be superior in reducing loneliness. Because loneliness is a primary risk factor for depression relapse in young people, we therefore expect the benefits of Groups 4 Health to be particularly apparent at 12-month follow up. DISCUSSION: This trial will be the first to evaluate an intervention that targets loneliness, in comparison to the current gold standard treatment approach - CBT. If found to be effective, this program offers a new approach to treatment and relapse prevention of depression among young people. TRIAL REGISTRATION: Trial prospectively registered on ANZCTR ( ACTRN12618000440224 ), registered on 27/03/2018.