ImportanceThere are many concerns about the link between social media use and adolescent mental health. However, most research has studied adolescents from the general population, overlooking clinical groups. To address this gap, we synthesize, quantify and compare evidence on the relationship between social media use and internalising symptoms in adolescent clinical and community samples.Data SourcesWe searched five electronic databases for peer-reviewed publications (MEDLINE, Web of Science, PsychInfo, Scopus) and preprints (Europe PMC) published in English between 2007 and 2022.Study SelectionWe included cross-sectional and longitudinal studies quantifying the relationship between social media use and internalising symptoms, excluding experimental studies and randomised controlled trials. Two blinded reviewers initially identified 7389 studies.Data Extraction and SynthesisWe adhered to the PRISMA and MOOSE guidelines for selection and reporting. The data was pooled using a random-effect model and robust variance estimation. Two reviewers independently assessed the quality of evidence with the Quality of Survey Studies in Psychology Checklist.Main Outcomes and MeasuresWe preregistered our hypotheses and primary study outcomes on PROSPERO (CRD42022321473). Articles were included if they reported at least one quantitative measure of a) social media use: time spent, active vs. passive use, activity, content, user perception, and other; and b) internalising symptoms: anxiety, depression or both.ResultsWe reviewed 127 studies including 1,061,293 adolescents and 775 effect sizes, of which only 8% examined clinical samples. In these samples, we found a positive, significant yet small meta-correlation between social media use and internalising symptoms, both for time spent (N= 2893,r= .08,95% CI= [.01, .15],p= .033,I2= 57.83) and user engagement (N = 859;r= .12,95% CI= [.09, .15],p= .002,I2= 82.67). These associations mirrored those in community samples.Conclusions and RelevanceWe highlight a lack of research on clinical populations, a critical gap considering that public concerns centre on the increase in youth mental health symptoms at clinical levels. This paucity of evidence not only restricts the generalizability of existing research but also hinders our ability to effectively evaluate and compare the link between social media use and youth mental health.