The purpose of this study was to establish the prevalence of mental disorders in juveniles who sexually offended (JSOs). A meta-analysis was performed based on studies reporting on the prevalence rates of mental disorders in JSOs. Furthermore, differences in mental disorders between JSOs and juveniles who offended non-sexually (non-JSOs) were assessed. In total, 21 studies reporting on mental disorders in 2951 JSOs and 18,688 non-JSOs were included. In the total group of JSOs, 69% met the criteria for at least one mental disorder; comorbidity was present in 44%. The most common externalizing and internalizing disorders were respectively conduct disorder (CD; 51%) and anxiety disorder (18%). Compared to non-JSOs, JSOs were less often diagnosed with a Disruptive Behavior Disorder (DBD, i.e., CD and/or Oppositional Deviant Disorder [ODD]), an Attention-Deficit/Hyperactivity Disorder (ADHD) and a Substance Use Disorder (SUD). No significant differences were found for internalizing disorders. In conclusion, although the prevalence of externalizing disorders is higher in non-JSOs, mental disorders are highly prevalent in JSOs. Even though results of the current meta-analysis may overestimate prevalence rates (e.g., due to publication bias), screening of JSOs should focus on mental disorders.