Posttraumatic stress disorder (PTSD) is highly comorbid with substance use disorders (SUDs), resulting in high prevalence of PTSD among individuals in residential SUD care. However, there is limited research on integrating trauma treatment into residential SUD care settings. The aim of the present project was to conduct an initial evaluation of the effects of group-based Written Exposure Therapy (WET) on PTSD and depressive symptoms that was integrated into programming for individuals in residential SUD treatment. Participants were 48 Veterans with comorbid PTSD-SUD from a 28-day residential SUD program at a Veterans Affairs Medical Center. Eligible participants were enrolled in 5 sessions of WET, delivered in twice-weekly in a group format. PTSD symptoms and depressive symptoms were assessed at each session with the Posttraumatic Stress Disorder Checklist, DSM-5 version (PCL-5) and the Patient Health Questionnaire (PHQ-9). Of the enrolled sample of 48 participants, 92% (n = 44) completed 3 sessions, while 56% (n = 28) completed 5 sessions. Dependent samples t-tests showed significant reductions from baseline in PTSD symptoms within-persons at both the 3-session (t(43) = 4.77, p < .001, d = .72) and 5-session mark (t(27) = 4.36, p < .001, d = .82). In addition, there were significant reductions in depressive symptoms after 3 sessions (t(38) = 3.01, p < .01, d = .48) and after 5 sessions (t(23) = 2.97, p < .01, d = .61). Findings demonstrate that brief, group-delivered WET shows promise for addressing PTSD and depressive symptoms in residential SUD treatment. Results of the present evaluation could inform further efficacy testing and implementation of PTSD treatment into residential SUD settings.