Carceral officials often cite diversion of medication for opioid use disorder (MOUD) (e.g., buprenorphine) as a reason for not offering MOUD treatment in jails and prisons with little understanding of patient perspectives. We aimed to understand patient perceptions of medication diversion from jail-based MOUD programs and the factors that contribute to and reduce diversion. We conducted thematic analyses of semi-structured interviews held in 2021-22 with 38 adults who received MOUD treatment and were released from eight Massachusetts jails that had implemented a MOUD program on or after September 2019. Consistent with prior reports from carceral staff, patients perceived MOUD diversion to happen less frequently than expected, which they attributed to dosing protocols that have effectively reduced it. Patients reported that MOUD availability reduced the contraband buprenorphine market, although other contraband substances have entered jails (fentanyl, oxycodone, K2). Patients perceived Subutex to have greater misuse potential and added diversion risks. Patients valued graduated consequences and other efforts to reduce MOUD diversion and contraband for making jails safer and for enabling patients to receive treatment. Nearly all participants reported having heard about, witnessed, or been involved in actual or attempted diversion, with variation in reports by jail. Patients suggested that dispensing MOUD to all who need it immediately upon intake would be the most effective way to reduce MOUD diversion and contraband.