We compared the race and ethnicity of individuals residing in states that did and did not expand Medicaid in 2014. Findings indicated that African Americans and Native Americans with substance use disorders who met new federal eligibility criteria for Medicaid were less likely than those of other racial and ethnic groups to live in states that expanded Medicaid. These findings suggest that the uneven expansion of Medicaid may exacerbate racial and ethnic disparities in insurance coverage for substance use disorder treatment.