When one sibling has autism spectrum disorder, the sibling relationship is often characterized by poorer quality with fewer interactions. Because sibling relationships provide a vital social framework for development, they have the capacity to be a risk or protective factor, depending on the quality of the relationship. One way to improve the quality of the sibling relationship is through typically developing sibling participation in a support group. In this study, researchers randomly assigned typically developing siblings to a 10-week support group or attention-only control group. Typically developing siblings in the support group showed significant improvements in the quality of their sibling relationship and interactions with their sibling with autism spectrum disorder compared to the control group. Autism spectrum disorder severity and externalizing behavior moderated the effects of the support group on positive affect. Findings suggest the positive impact of a support group on sibling relationships, a relationship that has the potential to be protective. Lay abstract The sibling relationship can be negatively impacted when one child has autism spectrum disorder. One way to improve the quality of that relationship is through typically developing sibling participation in a support group in which they learn about autism spectrum disorder and coping skills, develop a peer network, and discuss their feelings. Compared to participating in a similar group without a focus on autism spectrum disorder, siblings in the support group showed improvements in the quality of the sibling relationship. Findings suggest that sibling support groups can be a valuable resource to improve sibling relationship quality when one sibling has autism spectrum disorder.