School-based psychosocial interventions are increasingly put forward as a way to support young refugees’ and migrants’ well-being and mental health in resettlement. However, the evidence on these interventions’ effectiveness remains scarce and scholars denounce particular gaps in the evidence to date, pointing to a lack of large-scale, controlled studies and studies including social outcome measures. This cluster randomized study aims to strengthen the evidence base on school-based psychosocial interventions for refugee and migrant youth by assessing the effect of two interventions, Classroom Drama and Welcome to School, on youth’s mental health, resilience, and social relations in Belgium, Denmark, Norway, and the United Kingdom. Multilevel analyses were conducted separately for the two interventions (Classroom Drama, n = 307, ages 11-19; Welcome to School, n = 251, ages 11-23), using separate no-treatment control groups. Our analyses indicated a significant main, positive effect of Classroom Drama on perceived family support, and an effect on perceived support from friends that was moderated by country: in the United Kingdom, the intervention group reported an increase in perceived friend support, whereas the control group reported a decrease. Furthermore, baseline resilience moderated the effect of the Classroom Drama intervention on behavioral difficulties and well-being. No effects of Welcome to School on any of the outcome variables were found. Overall, this study provides novel, nuanced evidence on school-based psychosocial interventions for refugee and migrant adolescents.