ObjectiveThe aim of this review is to elucidate the characteristics of school-based mental health and psychosocial support (MHPSS) interventions in humanitarian contexts and the hypothesised mechanisms by which they influence well-being or learning outcomes.MethodsWe conducted a realist review and searched PubMed, Embase, Global Health, CINAHL, PsychInfo, PILOTS and grey literature through January 2022. Eligible studies included children age 6–12 years, were conducted in humanitarian contexts in low-income or middle-income countries, and focused on universal MPHSS prevention in an educational setting, using any study design. Data were extracted and analysed using narrative synthesis and realist analysis techniques to create ‘context–mechanism–outcome’ configurations that were iteratively developed to modify, refine and substantiate programme theories.ResultsTwenty-seven articles, representing 19 studies, were included in the review. We analysed data from 26 articles. Eleven evidenced-informed programme theories were developed at the levels of the child (n=4), teacher (n=3), caregiver (n=2), school environment (n=1) and school managers/administrators (n=1). At the child level, mechanisms related to strengthening coping skills, emotion regulation, interpersonal relationships led to improved psychosocial well-being or learning outcomes. At the teacher level, coping skills and the provision of support to students were linked to psychosocial well-being and learning outcomes. At the caregiver level, strengthening interpersonal bonds trigger improved psychosocial well-being, and at the school environment level, fostering feelings of security was linked to psychosocial well-being and learning outcomes. We did not find any evidence supporting the programme theory at the school managers/administrators level. We found limited evidence of positive impacts of the included interventions to support these programme theories.ConclusionsThese programme theories are a promising start towards ensuring school-based MHPSS interventions in humanitarian contexts better address the well-being and learning needs of children. Future research is needed to support these programme theories and enhance the evidence base.