BACKGROUND:Posttraumatic stress disorder (PTSD) results in high costs to society, particularly among military personnel. Much is known about PTSD treatments-such as exposure therapies-and their outcomes, but less is known about how treatment might impact social support and PTSD symptoms over the course of treatment. METHODS:In the current study, soldiers with PTSD (N = 162) were randomized to complete prolonged exposure therapy (either with or without virtual reality) or a waitlist control condition. We examined the impact of treatment on perceived social support as a secondary treatment outcome, as well as the associations between social support and PTSD symptoms over time. RESULTS:Exposure therapy increased perceived social support at the end of treatment compared to waitlist control, β = 0.43, 95% CI [0.13, 0.73]. Multigroup structural equation modeling using a cross-lagged panel design provided evidence that perceived social support was an antecedent of PTSD symptom improvement for participants engaging in treatment, but not for participants in the waitlist control. Treatment effects on change in PTSD symptoms was mediated by change in perceived social support (B = 1.10, 95% CI [0.20, 3.05]). LIMITATIONS:The results should be considered in light of limitations, including the characteristics of the sample of active duty soldiers, the measurement of social support, and missingess over the course of the study. CONCLUSIONS:These results suggest that increased perceived social support is a secondary outcome of exposure therapy and may be one pathway through which treatment reduces PTSD symptoms.