Past research has found intergroup contact to be a promising intervention to reduce prejudice and has identified adolescence as the developmental period during which intergroup contact is most effective. Few studies, however, have tested whether contact-based interventions can be scaled up to improve intergroup relations at a large scale. The present research evaluated whether and when the National Citizen Service, a large-scale contact-based intervention reaching one in six 15- to 17-year-olds in England and Northern Ireland, builds social cohesion among adolescents from different ethnic backgrounds. In a diverse sample of adolescents (N = 2099; Mage = 16.37, age range: 15-17 years; 58% female), this study used a pretest-posttest design with a double pretest to assess the intervention's effectiveness. Controlling for test-retest effects, this study found evidence that the intervention decreased intergroup anxiety and increased outgroup perspective-taking-but not that it affected intergroup attitudes, intergroup trust, or perceptions of relative (dis-)advantage. These (small) effects were greater for adolescents who had experienced less positive contact before participating and who talked more about group differences while participating. These findings suggest that the intervention might not immediately improve intergroup relations-but that it has the potential to prepare adolescents, especially those with less positive contact experiences before the intervention, for more positive intergroup interactions in the future.