Importance This systematic review and meta-analysis synthesized global longitudinal studies to estimate associations between social media, video games, and other digital media use with health and developmental outcomes in children and adolescents. Objective To provide a meta-analytic synthesis of evidence on digital media use and health and developmental outcomes among individuals aged 0 to 18 years. Data Sources This review was preregistered with PROSPERO, MEDLINE, PsycINFO, EMBASE, and ERIC databases and gray literature were searched from 2000 to 2024. Study Selection Inclusion criteria were English-language longitudinal studies of participants aged 0 to 18 years reporting quantitative associations between digital media use and health or developmental outcomes. Data Extraction and Synthesis Following PRISMA guidelines, 153 studies (115 cohorts, 1072 effect sizes) from 18 933 articles met criteria for quantitative synthesis. Random-effects meta-analyses estimated pooled correlations (r) with 95% CIs. Heterogeneity and moderators (age, sex, measurement method, follow-up duration, year of exposure) were examined. Study quality was assessed using the National Institutes of Health’s Quality Assessment Tool. These data were analyzed from February 2025 to August 2025. Main Outcomes and Measures Primary outcomes were socialemotional, cognitive, physical and motor health, and development associations. Results A total of 153 studies (115 cohorts, 1072 effect sizes) with 18 933 participants met criteria for quantitative synthesis. Study participant ages ranged from 2 to 19 (mean [SD], 12.81 [2.79]) years, with 53.8% female and 46.2% male. Most studies were conducted in Europe (62 [40.5%]) and North America (60 [39.2%]), followed by Asia (22 [14.4%]), Australia (5 [3.3%]), and Latin America (1 [0.7%]). Social media use was associated with higher depression, externalizing and internalizing behaviors, self-injurious thoughts, problematic internet use, and substance use ( r = 0.09; 95% CI, 0.06-0.12 to r = 0.21; 95% CI, 0.13-0.29) and also with lower academic achievement, poorer self-perception, and less positive development ( r = −0.14; 95% CI, −0.26 to −0.01 to r = −0.07; 95% CI, −0.11 to −0.02). Video gaming was associated with higher aggression and externalizing behaviors ( r = 0.16; 95% CI, 0.09-0.23 and r = 0.17; 95% CI, 0.07-0.26, respectively) and higher attention/executive functioning ( r = 0.10; 95% CI, 0.03-0.16). Other digital media use, including digital device use and messaging/communication media, was associated with depression ( r = 0.05; 95% CI, 0.00-0.09 to r = 0.12; 95% CI, 0.02-0.22). Associations between social media and depression were stronger in early adolescence (β = 0.09) and with self or parent-reported outcomes (β = 0.09); associations between social media and positive development were stronger with objective exposure measurement (β = 0.08). More recent social media exposure years showed stronger associations with substance use (β = 0.10). Conclusions and Relevance In this systematic review and meta-analysis, digital media use was consistently associated with risks to child and adolescent health and development, particularly for social media. These findings highlight the need for targeted, multifaceted policies and interventions to mitigate potential harms from digital media exposure.