Introduction: Problematic alcohol use is highly comorbid with posttraumatic stress disorder (PTSD), and prior work has demonstrated that individuals with PTSD may self-medicate with alcohol in an effort to reduce their symptoms. The combination of impulsivity and alcohol-related expectancies influences the development of problematic drinking patterns. When examining individuals diagnosed with PTSD, PTSD-related alcohol expectancies may be particularly relevant to the etiology of problematic drinking. To date, no studies have specifically examined PTSD-specific alcohol expectancies as they relate to alcohol use severity in a clinical sample. Methods: The current study examined the relationship between impulsivity, PTSD-related alcohol expectancies, and severity of alcohol use in a sample of 63 individuals diagnosed with comorbid PTSD and substance use disorders who were receiving treatment in a residential substance use treatment program. Results: Results indicated that PTSD-related alcohol expectancies moderated the relationship between impulsivity and alcohol use severity. Specifically, at low to moderate levels of positive PTSD-related alcohol expectancies, impulsivity significantly predicted alcohol use severity, while impulsivity had no impact on the prediction of alcohol use severity when such expectancies were high. Additionally, the relationship between impulsivity, expectancies, and alcohol use severity was significant for positive, but not negative, PTSD-related alcohol expectancies. Conclusions: Overall, these results suggest that impulsivity and PTSD-related alcohol expectancies interact to predict alcohol use severity in a comorbid PTSD and substance dependent sample.