Background: Adolescent mental health issues are a major public health concern, highlighted by the US Surgeon General as a crisis. Traditional school-based interventions show inconsistent success, creating a demand for effective solutions. Aims: This study evaluates the impact of technology-supported cooperative learning (CL) on adolescent mental health, focusing on positive peer relations and peer victimization. Materials and Methods: Participants included 813 adolescents (50.2% female; 70.7% White) from 12 middle and high schools in the Pacific Northwest. The study used hierarchical linear modelling (HLM) to assess the effectiveness of CL facilitated via PeerLearning.net. Results: Implementing CL led to significant improvements in peer relations and reductions in victimization and mental health problems, with moderate-to-large effect sizes observed across different demographics. Positive peer relations significantly predicted lower victimization and improved mental health. Discussion: The findings highlight the potential of technology-supported CL in addressing adolescent mental health by enhancing protective factors and reducing risks. Such interventions offer a scalable and sustainable approach for schools to address mental health challenges. Conclusion: Technology-supported cooperative learning offers a promising strategy for improving adolescent mental health, demonstrating significant benefits in peer relations and reducing victimization. This approach provides schools with an accessible and effective tool to tackle the mental health crisis among students.