BACKGROUND: Children and youth experiencing homelessness are highly affected by adverse childhood experiences (ACEs) and related health harms. We aimed to synthesise ACE prevalence in this population, examine whether study characteristics influenced prevalence estimates, and summarise associations of ACEs with health-related outcomes. METHODS: In this systematic review and meta-analysis, we searched MEDLINE, Embase, PsycINFO, the Cochrane Library, and Google Scholar for studies published from inception to Jan 23, 2026. We included quantitative studies published from Jan 1, 2000, of participants aged 24 years and younger experiencing homelessness that reported prevalence of individual ACE categories or cumulative ACE exposure (≥1 or ≥4 ACEs). Aggregate, study-level data were extracted by two researchers, and study quality was assessed with the JBI checklist for prevalence studies and an additional risk-of-bias tool. We used a generalised linear mixed-effects model for the meta-analysis; heterogeneity was assessed with I2, τ2, and 95% prediction intervals and explored through subgroup analyses and univariable meta-regression. The protocol was registered in PROSPERO (CRD42021233449). FINDINGS: We identified 20 745 records and included 47 studies. 45 studies reported prevalence for individual ACE categories and seven reported cumulative ACE exposure (≥1 or ≥4 ACEs). Meta-analysis was feasible for physical abuse (36 studies; n=10 240) and sexual abuse (39 studies; n=14 000). Pooled prevalence was 61·7% (95% CI 52·9-69·9) for physical abuse and 27·2% (95% CI 23·1-31·8) for sexual abuse. Heterogeneity was substantial (I2 97·6% for physical abuse and 95·7% for sexual abuse). Physical abuse prevalence was higher in studies recruiting from service and street or public settings than in service-only samples, and in those that used validated assessment instruments. Reported ACE-health associations were heterogeneous, with repeated links to poorer health outcomes. INTERPRETATION: This study provides the first quantitative synthesis of ACE prevalence in children and youth experiencing homelessness and indicates a high burden of abuse, with pooled estimates for physical and sexual abuse markedly higher than general-population estimates. Integrated multiagency services and ACE-focused prevention strategies are needed. FUNDING: None.