Background: Buprenorphine has been proven effective in treating opioid use disorder. However, the high cost of buprenorphine and the limited prescribing capacity may restrict access to this effective medication-assisted treatment for opioid use disorder. Objective: To examine whether Medicaid expansion and physician prescribing capacity may have impacted buprenorphine utilization covered by Medicaid. Research Design: We used a quasi experimental difference-indifferences design to compare the pre-post changes in Medicaid-covered buprenorphine prescriptions and buprenorphine spending between the 26 states that implemented Medicaid expansions under the Affordable Care Act in 2014 and those that did not. Subjects: All Medicaid enrollees in the expansion states and the nonexpansion and late-expansion states. Measures: Quarterly Medicaid prescriptions for buprenorphine and spending on buprenorphine from the Centers for Medicare and Medicaid Services Medicaid Drug Utilization files 2011 to 2014. Results: State implementation of Medicaid expansions in 2014 was associated with a 70% increase (P < 0.05) in Medicaid-covered buprenorphine prescriptions and a 50% increase (P < 0.05) in buprenorphine spending. Physician prescribing capacity was also associated with increased buprenorphine utilization. Conclusions: Medicaid expansion has the potential to reduce the financial barriers to buprenorphine utilization and improve access to medication-assisted treatment of opioid use disorder. Active physician participation in the provision of buprenorphine is needed for ensuring that Medicaid expansion achieves its full potential in improving treatment access.