An extensive body of research has demonstrated that patients with a co-occurring posttraumatic stress disorder (PTSD) and substance use disorder (SUD) diagnosis are at high risk for a wide range of negative clinical outcomes, including treatment noncompletion. However, no studies to date have explored the effect of a PTSD-SUD diagnosis on residential SUD treatment completion, as well as potential moderators of this effect. Consequently, the goal of this study was to examine the interactive effect of a PTSD diagnosis, distress tolerance (DT), and gender on residential SUD treatment retention. Participants were 214 substance-dependent patients consecutively admitted to a residential SUD treatment facility. Participants were administered diagnostic interviews, completed a laboratory-based measure of DT, and were followed throughout the course of treatment. Although no significant main effects were found, results did reveal a significant PTSD X gender X DT interaction. Post hoc analyses indicated that, among men, those with a current diagnosis of PTSD and low DT completed a significantly lower proportion of residential SUD treatment compared to all other groups. The implications of the study's findings for identifying ways to improve residential SUD treatment retention among patients with a PTSD-SUD diagnosis are discussed.