BACKGROUND: Current international recommendations to address the large treatment gap for mental healthcare in low- and middle-income countries are to scale up integration of mental health into primary care. There are good outcome studies to support this, but less robust evidence for effectively carrying out integration and scale-up of such services, or for understanding how to address contextual issues that routinely arise. AIMS: This protocol is for a process evaluation of a programme called Mental Health Scale Up Nigeria. The study aims are to determine the extent to which the intervention was carried out according to the plans developed (fidelity), to examine the effect of postulated moderating factors and local context, and the perception of the programme by primary care staff and implementers. METHOD: We use a theoretical framework for process evaluation based on the Medical Research Council's Guidelines on Process Evaluation. A Theory of Change workshop was carried out in programme development, to highlight relevant factors influencing the process, ensure good adaptation of global normative guidelines and gain buy-in from local stakeholders. We will use mixed methods to examine programme implementation and outcomes, and influence of moderating factors. RESULTS: Data sources will include the routine health information system, facility records (for staff, medication and infrastructure), log books of intervention activities, supervision records, patient questionnaires and qualitative interviews. CONCLUSIONS: Evidence from this process evaluation will help guide implementers aiming to scale up mental health services in primary care in low- and middle-income countries.