Opioid use disorder (OUD) is a notable concern in the United States (US) and strongly associated with mortality. There is a high prevalence of OUD in patients with posttraumatic stress disorder (PTSD) and the mortality associated with OUD may be exacerbated in patients with PTSD. Medication-assisted treatment (MAT) for OUD has become standard of care for OUD and has been shown to reduce mortality. However, there has been little study of MAT and mortality in patients with PTSD and OUD. We conducted a retrospective cohort study in U.S. veterans who had newly engaged in PTSD treatment, were diagnosed with OUD and were provided MAT for at least one day between 2004 and 2013. We assessed mortality for one year following the index diagnosis date. We calculated all-cause mortality as well as death by external cause, overdose plus suicide, overdose, and suicide rates per 100,000. We used hazard ratios (HR) and 95% confidence intervals (CI) to compare death rates between patients with high versus low adherence to MAT. We evaluated the impact of high versus low exposure to general substance abuse care. We considered a confidence interval that did not cross one to be significant. A total of 5,901 patients met inclusion criteria. Most patients were men and the average age was 43.3 years ( = 13.8). The all-cause mortality rate was 1,370 per 100,000 patients. High adherence to MAT resulted in a non-significant, decreased risk for death due to all-cause (HR = 0.73, 95% CI [0.47, 1.13]), external cause (HR = 0.71, 95% CI [0.38, 1.35]), and overdose or suicide (HR = 0.66, 95% CI [0.33, 1.35]). Patients with high exposure (≥ 60 days) to general substance abuse care were significantly less likely to die due to external cause (HR = 0.39, 95% CI [0.18, 0.85]) and overdose or suicide (HR = 0.31, 95% CI [0.12, 0.77]). In patients with PTSD and OUD, improved adherence to MAT and greater exposure to general substance abuse care may result in lower mortality. Studies with longer follow-up and larger sample sizes to assess the impact of MAT on suicide are needed to confirm our findings.