Although there have been numerous studies examining the prevalence of overweight and obesity among children and adolescents with intellectual disabilities, they have not yet been integrated and synthesized through a systematic quantitative review process. The purpose of this systematic review and meta-analysis was to determine: (i) the prevalence of overweight/obesity among children and adolescents with intellectual disabilities; (ii) the sources of heterogeneity in studies reporting the prevalence of overweight/obesity in this population; and (iii) the risk of overweight/obesity in this population compared with their typically developing peers. A systematic literature search was performed and 16 studies, published between 1985 and 2015, met the inclusion criteria. The resulting pooled prevalence estimates for overweight, overweight-obesity and obesity were respectively: (i) 15%, 30%, and 13%, in children; and (ii) 18%, 33%, and 15% in adolescents. Subgroup analyses showed significant variations in the pooled prevalence estimates as a function of geographical region, recruitment setting, additional diagnoses, and norms used to define overweight or obesity. The findings also showed adolescents with intellectual disabilities to be respectively 1.54 and 1.80 times more at risk of overweight-obesity and obesity than typically developing adolescents. Unfortunately, no such comparison is available for children. © 2016 World Obesity.