There is increasing evidence supporting the effectiveness of scalable mental health and psychosocial support (MHPSS) interventions delivered by non-specialists for improving wellbeing among migrant populations in humanitarian settings. Balancing fidelity in the implementation of evidence-based MHPSS interventions with their fit to the needs and preferences of new populations and contexts remains a challenge when introducing MHPSS interventions in new settings. This paper describes a community-based participatory approach to MHPSS intervention design incorporating processes to promote local adaptability and fit while maintaining standardized elements of existing MHPSS interventions.