Objective To examine county‐level geographic variation in treatment admissions among opioid treatment programs (OTPs) that accept Medicaid in the continental United States. Data Sources/Study Setting Data come from the 2012 National Survey of Substance Abuse Treatment Services. Study Design/Data Collection We used local measures of spatial autocorrelation (LISA) analysis to identify (1) clusters of counties with higher and lower than average rates of opioid use disorders and (2) clusters of counties with higher and lower than average treatment admissions among OTPs that accept Medicaid, adjusting for county population size. Principal Findings Our results reveal several clusters of counties with higher than average rates of opioid use disorder (OUD) and lower than average treatment admissions among OTPs that accept Medicaid. These clusters are highly concentrated in the Southeast region of the country and include Arkansas, Kentucky, Louisiana, Mississippi, and Tennessee. Conclusions Medicaid enrollees in areas in the Southeast have the largest gaps between county‐level OUD rates and estimated county‐level capacity for treatment, as measured by county‐level total treatment admissions among OTPs that accept Medicaid. Policy makers should consider strategies to increase the availability of OTPs with the capacity to serve Medicaid enrollees.