Residential Substance Use Disorder Treatment Act of 2021 This bill revises and reauthorizes through FY2026 grants for residential substance use disorder treatment programs at state and local correctional and detention facilities. Among the revisions, the bill replaces statutory references to substance abuse with substance use disorder , specifies that the term residential substance use disorder treatment program includes a medication-assisted treatment program, requires the chief medical officer or other staff overseeing a program to complete training on the science of addiction and the latest research and clinical guidance on treating substance use disorders in criminal justice settings, and allows people who are awaiting trial or in pretrial detention to participate in the programs.