Little is known about the correlates of use of medications to treat opioid use disorder (MOUD, ie, buprenorphine, methadone, naltrexone) offered in jails. We evaluated the implementation and outcomes of a MOUD program offered by 2 of the first jails nationwide to provide access to such care. At jail entry, 48.7% of individuals with opioid use disorder were being treated with MOUD. During incarceration, 65.1% received MOUD, attributable to a 9.2% increase in use of methadone (from 15.9% to 25.1%) and a 10.1% increase in use of buprenorphine (from 28.5% to 38.6%). During incarceration, 32.3% of individuals were continued on the same MOUD from the community, 25.4% were started, 8.9% stopped, and 7.5% switched type. A total of 25.9% entered jail not on any MOUD and were not started on it. Use of MOUD during incarceration was positively associated with having received MOUD in the community (odds ratio, 12.2; 95% confidence interval, 5.8-25.5) and incarceration at site 1 compared with site 2 (OR, 24.6; 95% CI, 10.9-54.4). Expanded access to MOUD in jails can engage an at-risk population with treatment. Understanding factors related to this population’s use of MOUD may aid efforts to optimize care during incarceration and after community re-entry.