The Reforming and Expanding Access to Treatment Act or the TREAT Act This bill amends title XIX (Medicaid) of the Social Security Act to allow federal payment for qualified substance use disorder services furnished to inmates in public institutions under state Medicaid programs. "Qualified substance use disorder services" include medication or behavioral treatment administered to eliminate, mitigate, treat, or cure an individual's substance abuse disorder or use or abuse of drugs. The Substance Abuse and Mental Health Services Administration shall not establish, maintain, or implement any policy that restricts its statutorily authorized provision or support of substance abuse treatment for incarcerated individuals.