This study evaluated the effects of a school-based creative expression program on mental health and classroom social relationships in elementary school children with refugee and nonrefugee migration backgrounds. It was hypothesized that children receiving the intervention would report less externalizing and internalizing problem behaviors, less posttraumatic functioning, and more positive classroom social relationships at posttest than children receiving education as usual, particularly for refugee children. Classes in three multiethnic Belgian elementary schools were randomly assigned to a creative intervention (7 classes, 68 students) or control condition (6 classes, 52 students). All participants (8-12 years old) had a migration background. Almost half (47%) were refugees, 53% were first- to third-generation nonrefugee immigrants. Data collection included pre- and posttest assessment with children, parents, and teachers. Multilevel analysis was used to assess outcomes. Children in the intervention condition rated the classroom climate at posttest more positive than their control peers (d = .33). Children who received the intervention did not show less symptomatic functioning than children in the education-as-usual condition. However, post hoc analysis by baseline severity showed that students with high baseline levels of posttraumatic stress reported less trauma symptoms at posttest in the intervention group than in the control group (d = -.97). This effect was moderated by children's refugee background, indicating a differential effect in which refugee children show more reduction of trauma symptoms as compared to nonrefugee immigrant children. The intervention supported classroom climate and alleviated posttraumatic stress in children with increased posttraumatic symptomatology. (PsycInfo Database Record (c) 2022 APA, all rights reserved).