OBJECTIVE: This study examined the association between autism spectrum disorder (ASD) symptom severity and academic outcomes and classroom functioning in a community-based sample of children with and without ADHD. METHOD: Participants included children with ADHD (n =179) and a non-ADHD group (n =212). ASD symptom severity, academic and learning skills, and classroom functioning were assessed via teacher report using the Social Skills Improvement System (SSIS; ASD and Academic Competence subscales) and the Strengths and Difficulties Questionnaire (SDQ; all subscales). RESULTS: Children with ADHD had higher teacher-reported ASD symptoms than children without ADHD (beta= .62, p< .001). Greater teacher-reported ASD symptoms were associated with more behavioral difficulties in the classroom for children with ADHD (beta= .50, p< .001). There was little evidence of an association between academic competence and ASD symptom severity in children with ADHD (beta= -.11, p = .15). CONCLUSION: ASD symptoms are associated with elevated classroom behavioral difficulties for children with ADHD.