Objective: Although autism spectrum disorder (ASD) symptoms are associated with poorer functioning in children with attention-deficit/hyperactivity disorder (ADHD), it is unclear which ASD symptom domains are most impairing. This study investigated whether specific ASD symptom domains were associated with child functioning in children with ADHD. Method: Parents of 164 children with ADHD completed a diagnostic interview to assess ADHD and comorbidities. Parents reported on ASD symptoms (Social Communication Questionnaire) and child quality of life (Pediatric Quality of Life Inventory 4.0). Parents and teachers completed the Strengths and Difficulties Questionnaire (emotional, conduct, and peer problems). Results: Repetitive and stereotyped behaviors were independently associated with emotional ( p = .02) and conduct ( p = .03) problems, and poorer quality of life ( p = .004). Reciprocal social interaction deficits were independently associated with peer problems ( p = .03). Conclusion: Reciprocal social interaction deficits and repetitive and stereotyped behaviors are important areas that should be focused on in ADHD assessment and treatment.