Background: Subjects with intellectual disability (ID) are vulnerable to experience psychiatric disorders. The present authors performed a systematic review and meta-analysis to estimate the prevalence of co-occurring psychiatric disorders, excluding co-occurring autism spectrum disorders, in subjects with intellectual disability. Method: The present authors performed a random-effects meta-analysis of the prevalence of psychiatric disorders in adults and adolescents with intellectual disability. Results: Twenty-two studies were included. The pooled prevalence of any co-occurring psychiatric disorders in intellectual disability was 33.6% (95% CI: 25.2%-43.1%) with high heterogeneity but no publication bias. Prevalence was lower in population-based studies, in studies that used ICD criteria for the psychopathology and in studies with low risk of bias. The prevalence was higher in mild, moderate and severe intellectual disability than in profound intellectual disability. Conclusions: Psychiatric disorders are common in subjects with intellectual disability, and the present authors found that clinical and methodological moderators affect the pooled prevalence.