Objective: Sleep problems are common in children with autism spectrum disorders (ASD) and ADHD and impact adversely on child and parent well-being. The study evaluated the efficacy of a brief behavioral sleep intervention in children with comorbid ADHD–ASD. Method: A subsample of children with ADHD–ASD (n = 61; 5-13 years; 89% male) participating in the Sleeping Sound With ADHD study were included in the current investigation. The subsample comprised of 28 children randomized to the sleep intervention group, while 33 were randomized to usual clinical care. The intervention consisted of two clinical consultations and a follow-up phone call covering sleep hygiene and standardized behavioral strategies. Results: Children with ADHD–ASD who received the intervention had large improvements in sleep problems and moderate improvements in child behavioral functioning 3 and 6 months post-randomization. Conclusion: These findings suggest that a brief behavioral sleep intervention can improve sleep problems in children with ADHD–ASD.