IntroductionMental healthcare is one of the biggest challenges for healthcare systems. Comorbidities between different mental disorders are common, and patients suffer from a high burden of disease. While the effectiveness of collaborative and stepped care models has been shown for single disorders, comorbid mental disorders have rarely been addressed in such care models. The aim of the present study is to evaluate the effectiveness of a collaborative and stepped care model for depressive, anxiety, somatoform and alcohol use disorders within a multiprofessional network compared with treatment as usual.Methods and analysisIn a cluster-randomised, prospective, parallel-group superiority trial, n=570 patients will be recruited from primary care practices (n=19 practices per group). The intervention is a newly developed collaborative and stepped care model in which patients will be treated using treatment options of various intensities within an integrated network of outpatient general practitioners, psychiatrists, psychotherapists and inpatient institutions. It will be compared with treatment as usual with regard to effectiveness, cost-effectiveness and feasibility, with the primary outcome being a change in mental health-related quality of life from baseline to 6 months. Patients in both groups will undergo an assessment at baseline, 3, 6 and 12 months after study inclusion.Ethics and disseminationThe study has been approved by the ethics committee of the Hamburg Medical Association (No. PV5595) and will be carried out in accordance with the principles of the Declaration of Helsinki. For dissemination, the results will be published in peer-reviewed journals and presented at conferences. Within the superordinate research project Hamburg Network for Health Services Research, the results will be communicated to relevant stakeholders in mental healthcare.Trial registration numberNCT03226743.