The opioid epidemic has claimed the lives of more than 183,000 individuals since 1999 and is now the leading cause of accidental death in the United States. Meanwhile, rates of incarceration have quadrupled in recent decades, and drug use is the leading cause of incarceration. Medication-assisted treatment or MAT (i.e. methadone, buprenorphine) is the gold standard for treatment of opioid use disorder. Incarcerated individuals with opioid use disorder treated with methadone or buprenorphine have a lower risk of overdose, lower rates of hepatitis C transmission, and lower rates of re-incarceration. Despite evidence of improved outcomes, many jails and prisons do not offer MAT to individuals with opioid use disorder. This seems partly due to a scientifically unjustified preference for an abstinence-only treatment approach. The absence of MAT in prisons and jails results in poor outcomes for individuals and poses a public health threat to communities. Furthermore, it disproportionately harms poor communities and communities of color. Health care providers in prisons and jails have an ethical obligation to offer MAT to individuals with opioid use disorder to mitigate risk of infectious diseases, opioid overdose and health disparities associated with incarceration.