Social skills group training is frequently used to support autistic children and adolescents in coping with everyday social demands and expectations. Meta-analytic studies suggest small to medium effect sizes. Effect moderators must be better understood to inform implementation and allow optimization of intervention protocols. Using data from two pragmatic randomized trials (N = 241) of the KONTAKT™ social skills group training program as an add-on to standard care in clinical settings, we aimed to explore the moderating role of co-occurring attention-deficit hyperactivity disorder in children (7-12 years) and adolescents (13-18 years). Two distinct response criteria were defined based on the parent-rated Social Responsiveness Scale: reliable improvement ([greater than or equal to] 25 points) and clinically relevant improvement ([greater than or equal to] 10 points). Moderator analyses indicated that the intervention effect was moderated by co-occurring attention-deficit hyperactivity disorder and age group. Logistic regressions stratified by co-occurring attention-deficit hyperactivity disorder showed significant effects among those without attention-deficit hyperactivity disorder (n = 63), both in terms of reliable improvement (odds ratio: 11.85, p = 0.002) and clinically relevant improvement (odds ratio: 10.00, p = 0.001). In contrast, no significant effects were observed in those with co-occurring attention-deficit hyperactivity disorder (n = 178). Explorative analyses further stratified by age suggested significant effects in adolescents, but not children, with co-occurring attention-deficit hyperactivity disorder. The findings preliminarily suggest that autistic children with attention-deficit hyperactivity disorder may need other or additional support.