Extracorporeal membrane oxygenation (ECMO) supports severe cardiorespiratory failure but carries a significant bleeding risk. This systematic review aims to report bleeding definitions in studies involving neonates and children treated with ECMO and to determine how heterogeneity in bleeding definition affects bleeding prevalence. We conducted a meta-analysis including all studies from database inception until April 26, 2024. We evaluated the description of bleeding definitions and the reported prevalence of bleeding on ECMO. Of 6,482 screened studies, 169 were included (n = 154,046 subjects). There were 69 distinct bleeding definitions; intracranial hemorrhage (n = 42 studies) and the Extracorporeal Life Support Organization (ELSO) definition (n = 35 studies) were the most common bleeding definitions. The need for intervention, including transfusions, was included in bleeding definitions in 57% of the studies. The overall pooled prevalence of bleeding was 33%. Excluding studies that reported solely intracranial bleeding, the pooled prevalence of bleeding was 41% in studies using the ELSO definition and 39% in studies using other definitions, with high heterogeneity. Variations in bleeding definitions may account for the variability in the reported prevalence of bleeding in children on ECMO. Furthermore, variability in clinical practices regarding interventions to control bleeding may affect estimates of the prevalence of bleeding.