We examine the early effects of U.S. state Medicaid expansions under the Affordable Care Act (ACA) on substance use disorder (SUD) treatment utilization. We couple administrative data on admissions to specialty SUD treatment and prescriptions for medications used to treat SUDs in outpatient settings with a differences-in-differences design. We find no evidence that admissions to specialty treatment changed in expanding states relative to non-expending states. However, post expansion, Medicaid-reimbursed prescriptions for medications used to treat SUDs in outpatient settings increased by 33% in expanding states relative to non-expanding states. Among patients admitted to specialty SUD treatment, we find that in expanding states Medicaid insurance and use of Medicaid to pay for treatment increased by 58% and 57% following the expansion. In an extension to the main analyses we find no evidence that the expansions affected fatal alcohol poisonings or drug-related overdoses. Overall, our findings provide evidence on the early effects of the ACA on SUD treatment utilization with the newly-eligible Medicaid population.