Nonmedical prescription sedative/tranquilizer (e.g., benzodiazepines) use (NMSTU) increases risk of overdose when combined with opioids and/or alcohol. Yet, little is known about NMSTU among those with alcohol and opioid use disorders. We aimed to characterize NMSTU and sedative/tranquilizer use disorder among adults with alcohol use disorder (AUD) and/or opioid use disorder (OUD) in a general population sample. We conducted analyses of 2008-2014 National Survey on Drug Use and Health data; adults with past-year AUD-only (n = 27,416), OUD-only (n = 2142), and co-occurring AUD and OUD (n = 1483) were included (total N = 31,041). Multivariable logistic regression models were utilized to examine correlates of past-month NMSTU and past-year sedative/tranquilizer use disorder. Focal independent variables were polysubstance use (i.e., number of substances used in the previous year) and psychiatric distress. Among those with AUD-only, 27.1% reported lifetime NMSTU, 7.6% reported past-year NMSTU, 2.7% reported past-month NMSTU, and 0.6% met criteria for past-year sedative/tranquilizer use disorder. Corresponding prevalence rates among those with OUD-only were 69.5%, 43.0%, 22.6%, and 11.3%. Those with co-occurring AUD and OUD displayed the highest rates of NMSTU (e.g., 27.5% with past-month NMSTU) and sedative/tranquilizer use disorder (20.2%). Across groups, more severe polysubstance use and psychiatric distress were associated with increased risk of NMSTU and sedative/tranquilizer use disorder. Results of this analysis indicate that >25% of adults with AUD and approximately 70% of those with OUD report lifetime NMSTU. Among these populations, individuals with more polysubstance use and greater psychiatric distress might benefit from targeted interventions to reduce NMSTU.