Posttraumatic stress disorder (PTSD) has been linked to problematic alcohol use. Those with PTSD have greater difficulty regulating emotions, which may help to explain the PTSD–drinking co-occurrence. However, emotion regulation as a mediator of PTSD–alcohol associations has been limited. In the present study, we examined this association. College students (N = 466, Mage = 19.5, 53% female) were assessed for PTSD by structured clinical interview, and were categorized into 3 groups: those who had not experienced a significant trauma (No Trauma, n = 182), those who had experienced a significant trauma but did not have current PTSD symptoms (Trauma Only, n = 171), and those with significant trauma and with current PTSD (partial or full; PTSD, n = 113). Alcohol use over the past 6 months and emotion regulation were assessed via self-report (Daily Drinking Questionnaire, Collins, Parks, & Marlatt, 1985; Difficulties in Emotion Regulation Scale, Gratz & Roemer, 2004). Findings revealed that those with trauma and clinically significant PTSD reported greater difficulty with emotion regulation than those who had not been exposed to trauma, which in turn significantly predicted alcohol use. This mediating effect was not found in those with trauma exposure alone, suggesting an important role for PTSD in this pathway. Findings also indicated that the ability to control emotion-based impulses is a particularly relevant mediating facet of emotion regulation. These results implicate emotion regulation as a potential explanatory link between PTSD and alcohol use, lending further support to the inclusion of emotion regulation training in PTSD treatment.