The prevalence of opioid use disorder (OUD) is especially high among individuals supervised by the criminal justice system (CJS), and a preponderance of opioid overdose deaths occur among individuals who have a history of CJS involvement (Binswanger et al., 2007; Bird et al., 2016). Because of the high prevalence of opioid use among CJS populations, jails offer a unique opportunity to identify and treat persons with OUD. In 2017, the FDA approved an extended-release formulation of buprenorphine (XR-B, Sublocade®; Indivior, Inc.) with subcutaneous injections of 100mg or 300mg buprenorphine delivered every four weeks. Early evaluations of XR-B in community setting show relatively high retention and positive opioid-use outcomes (Andorn et al., 2020; Ling et al., 2020). Current practice for delivering extended-release medication for opioid use disorder (MOUD) in jail settings is to detoxify the patient at admission, then provide an injection shortly before release (Vestal, 2018) with the targeted goal of reducing risk of relapse, overdose, and associated problems in the community. This focus on the post-release phase is widespread. In fact, all of the jail-based MOUD studies we located in literature searches for this proposal focused on initiating MOUD treatment shortly before jail/prison discharge and tracking community-based outcomes following release (e.g., continued care, relapse, and recidivism; see Moore et al., 2019). The ubiquitous focus on post-incarceration raises an important health-services question: Could there be benefits of initiating XR-B treatment at the time of jail admission, relative to the standard approach of detoxifying at admission, re-establishing opioid tolerance (with a week or more of administering oral buprenorphine), and initiating XR-B shortly before discharge? To test this, we propose a 3-year, two-site, randomized controlled trial in which new jail admissions (N=160) with OUD and sentences of < 6 months are assigned to either begin XR-B treatment immediately (experimental group) or 3-4 weeks prior to discharge (treatment as usual; TAU), including a lead-in sublingual Suboxone phase of 1-2 weeks. To assess the potential effects of in-jail XR-B maintenance, all study participants will be enrolled at jail admission and assessed monthly over their incarceration term. In-jail assessments include psychological distress (Kessler 6), opioid craving (VAS), and administrative records of in-jail behavioral infractions (administrative, serious, and related to diversion). Our inclusion of in-jail outcomes will allow us to have monthly (pre-release) follow-up data for nearly 100% of the sample, overcoming the common problem of missing data in CJS studies of MOUD. Post-release outcomes will be assessed 1 & 3 months following release from jail, and include XR-B continuation in the community (Sublocade is on the Medicaid formulary in Massachusetts), patient-centered perceived progress in recovery (TEA), opioid use (ASI, urine toxicolo...