Background <p>The prevalence of depressive symptoms among youth seems to be increasing. Depression among youth is associated with poorer academic performance and social relationships. Preventive interventions for youth with elevated depressive symptoms are essential. The STARr training was found to reduce depressive symptoms in youth; however, it was too linguistic and could be enhanced in terms of gender and cultural background. This study will examine whether a simplified, adapted modular CBT training (STARr 2.0) is effective in reducing depressive symptoms and co-occurring anxiety symptoms among youth with depressive symptoms. Further, this study will assess whether the module sequence preferences of adolescents, trainers, or data predict the outcome depressive symptoms. Additionally, alliance, motivation, and expectation of the treatment will be assessed as predictors. Last, we will examine moderators (age, gender) and change mechanisms (problem solving, cognitive restructuring, behavioral activation, and stress).</p> Methods <p>An n-of-1 study with a single-case observational design with a baseline and effect phase will be employed. The baseline phase starts three weeks before the training. We aim to include 30 adolescents between 12 and 20 years at two mental health organizations. All participants receive the adapted STARr-training in groups of six to eight participants. The training consists of 12 hourly sessions provided by a trained trainer. Participants receive in total three short questionnaires per week and six longer questionnaires (start baseline, before the start of the training, and after each module). The primary outcome is depressive symptoms and secondary outcomes are anxiety, stress, satisfaction with the training, and individual and societal functioning.</p> Discussion <p>The findings of this study will provide insight into whether an adapted simplified version of the STARr training is effective in reducing depressive symptoms and will also provide insight into change mechanisms and potential subgroup differences. If the adapted version is effective in reducing depressive symptoms, then the training can be used widely by mental health professionals.</p> Trail registration <p>Overview of Medical Research in the Netherlands (<i>OMON).</i> Registration number NL-OMON58273, registration date: 4 December 2025.</p>