Longitudinal studies across the globe documented significant increases in psychological stress and mental health problems among adolescents during the COVID-19 pandemic. Health concerns, school disruptions, and social disconnection were major sources of stress. High levels of perceived stress predicted worse mental health outcomes, with girls, older adolescents, and socio-economically marginalized youth experiencing more pronounced mental health deteriorations. However, social support from family and peers was a protective factor against increased stress and accompanying mental health problems. We urge policymakers and other key decision-makers to improve the availability and financing of mental health services and support programs for adolescents to address the wave of mental health challenges following the pandemic.