IntroductionTo control the spread of COVID-19, mitigation strategies have been implemented globally, which may have unintended harmful effects on child and adolescent mental health. This study aims to synthesise the indirect mental health impacts on children and adolescents globally due to COVID-19 mitigation strategies.MethodsWe included relevant reviews from MEDLINE, Embase, PsycINFO, LILACS, CINAHL, The Cochrane Library and Web of Science until January 2022 that examined the impact of COVID-19-related lockdown and stay-at-home measures on the mental health of children and adolescents. Data extraction and quality assessments were completed independently and in duplicate by BC and LH. A Measurement Tool to Assess Systematic Reviews-2 was used to assess the methodological quality.ResultsEighteen systematic reviews, comprising 366 primary studies, found a pooled prevalence of 32% for depression (95% CI: 27 to 38, n=161 673) and 32% for anxiety (95% CI: 27 to 37, n=143 928) in children and adolescents globally following COVID-19 mitigation measures. Subgroup analyses also uncovered important differences for both depression and anxiety by World Health Organization regions with few studies from Africa and relative high burden of anxiety and depression in the Eastern Mediterranean region.ConclusionsOur findings reveal a high prevalence of depression and anxiety in children and adolescents during the COVID-19 pandemic, globally, compared with prepandemic estimates. These findings highlight the urgency for governments and policymakers to strengthen mental health systems in the COVID-19 recovery, especially in low-and middle-income countries where compounding psychological stress, access and affordability of care and discrepant reporting of mental health in this population remains a challenge. We also provide insight into how to alter mitigation strategies to reduce the unintended negative consequences for the health and well-being of children and adolescents in future pandemics.PROSPERO registration numberCRD42022309348.