Typically developing siblings of a child with autism spectrum disorder may be at increased risk of mental health difficulties. A support group is one approach to improve mental health outcomes for typically developing siblings. During support groups, typically developing siblings discuss their feelings, learn coping strategies and problem-solving skills, and develop a peer network. We conducted a randomized controlled trial comparing a support group to an attention-only social control group. Some areas of mental health improved. Autism spectrum disorder symptom severity in the sibling with autism spectrum disorder moderated effects. Findings suggest continuing to examine what areas of mental health and adjustment are improved with support groups and subgroups of typically developing siblings for whom support groups might be particularly effective.