The nonmedical use of benzodiazepines-defined as use without a prescription or at a dose or frequency higher than prescribed-is increasing among adults in substance use disorder treatment and is associated with risk for overdose. The aim of the current study was to characterize the prevalence of nonmedical benzodiazepine use among adults seeking treatment for alcohol use disorder and to examine whether nonmedical benzodiazepine use was associated with: (1) polysubstance use and (2) greater anxiety sensitivity. A sample of 461 treatment-seeking adults with alcohol use disorder who were recruited for a cross-sectional study were included in this analysis. A total of 89 participants (19%) reported nonmedical benzodiazepine use in the previous 30 days. Results of a logistic regression indicated that polysubstance use (number of substances used in the past month) was associated with nonmedical benzodiazepine use. The association between anxiety sensitivity and nonmedical benzodiazepine misuse was moderated by gender; anxiety sensitivity was associated with benzodiazepine use among women, but not men. These results replicate findings from research on opioid use disorder suggesting that anxiety sensitivity is associated with nonmedical benzodiazepine use in women and not men. Targeted intervention to those with polysubstance use-including education on overdose risk when benzodiazepines are combined with other substances-is indicated in men and women with alcohol use disorder. Anxiety sensitivity may be a potential therapeutic target to reduce nonmedical benzodiazepine use among women with alcohol use disorder. (Am J Addict 2018;27:485-490).