Abstract Background Humanitarian crises increase the burden of mental disorders due to exposure to traumatic events and ongoing daily stressors. Effective mental health and psychosocial support (MHPSS) interventions exist, but barriers and facilitators for scaling up those interventions are less understood. The study aim was to identify barriers and facilitators for scaling up MHPSS interventions for populations affected by humanitarian crises in low- and middle-income countries. Methods A systematic review following PRISMA guidelines was conducted. Types of scale up were summarised, and barriers and facilitators analysed using the World Health Organization’s Expandnet framework of scaling up. Evidence quality was appraised using the Mixed Methods Appraisal Tool. Results Fourteen eligible studies were identified. Most described horizontal types of scale up, integrating services into primary and community care through staff training, task-sharing, and establishing referral and supervision mechanisms. Barriers were reported in a range of framework elements, but primarily related to those in the health system. The overall quality of studies were limited. Conclusion Few MHPSS interventions in humanitarian crises appear to have been scaled up, and scaling up efforts were largely horizontal which challenges long-term sustainability. Greater focus should be on both horizontal and vertical scaling up, which should be accompanied by higher quality research.