Background: Several states have passed Medicaid Home and Community-based Services (HCBS) waivers that expand eligibility criteria and available services for children with autism spectrum disorder (ASD). Previous research has shown considerable variation in these waivers, but little is known about the extent to which they address the health care needs of children with ASD. Objective: To determine the effects of Medicaid HCBS waivers, and their characteristics, on unmet health care needs among children with ASD. Methods: We used data from the 2003, 2007, and 2011 waves of the National Survey of Children's Health with detailed information on the Medicaid HCBS waiver programs of 35 states. Quasi-difference-in-difference-in-differences models were used to determine the effects of waivers and their characteristics on parent report of unmet health care needs of children with ASD compared with children without ASD. Results: Greater waiver cost limits per child, estimated costs of services, and enrollment limits were associated with significant decreases in the adjusted rate of unmet health care needs, with considerable variation by household income level. Conclusions: These findings suggest that Medicaid HCBS waivers significantly decrease the unmet need for health care among children with ASD, most substantially among those who would not otherwise qualify for Medicaid. The findings regarding the effects of specific aspects of these waivers can inform the development of insurance policies in other states to address the needs of children with ASD.