BackgroundMental health problems represent a large and growing public health concern. Primary care handles most of the patients with mental health problems, but there are many barriers to detection and treatment in this setting, causing under-recognition and under-treatment of patients. The service delivery model Primary Care Behavioral Health (PCBH) shows promise to manage mental health problems in primary care, but more research is needed regarding its effects on multiple levels.MethodsThis project investigates the effectiveness and implementation of a large-scale implementation of PCBH in Region & Ouml;sterg & ouml;tland, Sweden. The aim is to generate new knowledge concerning the impact of a real-world implementation and use of PCBH in routine primary care. A Pragmatic Stepped-Wedge Cluster Trial will be used: 24 PCBH primary care centres in one region will be compared with 48 standard care centres in three other regions. The model will be implemented sequentially at the PCBH centres according to a staggered timetable. Results will be investigated at patient, staff and organization levels and various forms of data will be collected: (1) local and national registry data; (2) questionnaire data; (3) interview data; and (4) document data.DiscussionThis project investigates the effectiveness and implementation of PCBH in routine primary care. The project could result in improved mental health care for the included patients and contribute to the general good for a wider population who have mental health problems. The project's study design will make it possible to assess many important effects of the PCBH service delivery model at different levels, providing evidence of the effectiveness (or not) of the PCBH model under routine conditions in primary care. The project has the potential to generate clinically meaningful results that can provide a basis for decisions concerning further implementation and use of the model and thus for future development of mental health care provision in primary care.Trial registrationNCT05633940, date of registration: 2021-04-21.