Children with autism spectrum disorders often exhibit comorbid behavioral problems. These problems have an impact on the severity of the core symptoms, the progression of the disorder as well as on the families' quality of life. We evaluated the effectiveness of the Stepping Stones Triple P group parent training program as a supplementary intervention in the treatment of children with autism spectrum disorder. Therefore, we employed a single group repeated measures design and assessed child variables via parents' and teachers' judgments at four successive time points. The participants were parents of 24 children with autism spectrum disorder aged between 3.6 and 12 years. We found a significant reduction of comorbid behavioral problems in the children, primarily in the parents' judgment at follow-up. Furthermore, a reduction of the autism spectrum disorder core symptoms emerged. The teachers' judgment particularly revealed an improvement in children's social relationships. Effect sizes were large (eta-squared ranging from 0.14 to 0.23). The findings demonstrate the effectiveness of the Stepping Stones Triple P as a supplementary intervention for reducing comorbid behavioral problems in the treatment of children with autism spectrum disorder. Higher parental self-efficacy and parental attributions, including parents' ability to influence child problem behaviors, are discussed as important factors for the effectiveness of Stepping Stones Triple P.