Abstract: OBJECTIVE: Trauma-informed guilt reduction therapy (TrIGR), a six-session cognitive behavioral therapy targeting trauma-related guilt and distress, reduces guilt and symptoms of posttraumatic stress disorder (PTSD) and depression, yet little is known regarding how and why TrIGR may be effective. METHOD: This study examined treatment-related changes in avoidant coping and trauma-related guilt cognitions as possible mediators of treatment effects on PTSD and depression outcomes at 3- and 6-month follow-up. Data were from a randomized controlled trial for treatment of trauma-related guilt comparing TrIGR and supportive care therapy among 145 post-9/11 US veterans (M(age) = 39.2 [8.1], 93.8% male). RESULTS: At pretreatment, most (86%) met PTSD criteria. Intent to treat analyses using parallel mediation models indicated changes in guilt cognitions, but not avoidant coping, mediated the effect of TrIGR on reducing PTSD severity at 3-month (a × b = -0.15, p < 0.01, 95% CI: [-0.24 to -0.06], p = 0.001) and 6-month (a × b = -0.17, 95% CI: [-0.26 to -0.07], p = 0.001) follow-up. Similarly, changes in guilt cognitions, but not avoidant coping, mediated the effect of TrIGR on reducing depression severity at 3-month (a × b = -0.10, 95% CI: [-0.18 to -0.02], p = 0.02) and 6-month (a × b = -0.11, 95% CI: [-0.20 to -0.03], p = 0.01) follow-up. CONCLUSIONS: Compared to guilt cognitions, changes in avoidant coping were less integral to downstream PTSD and depression symptom reduction. Guilt cognition change may be a salient active ingredient of PTSD and depression treatment for those with trauma-related guilt and a key therapy element to which providers should be attuned.