Introduction: Military service increases the risk of exposure to trauma, including military combat exposure (MCE) and military sexual trauma (MST). Both MCE and MST have individually been shown to increase the likelihood of developing posttraumatic stress disorder (PTSD), a potentially disabling mental illness. Aim: The aim was to examine the existing literature regarding the synergistic effect of MST and MCE in predicting PTSD symptoms and severity among military veterans. Method: A systematic review was conducted with the search criteria set to peer-reviewed papers that examined the synergistic effect of MST and MCE on PTSD, published in the English language between 2011 and 2022. Four electronic databases were searched (PsycINFO, PubMed, CINAHL, and Embase). Results: The search identified 1,334 studies, five of which were eligible for inclusion. All were conducted in the United States and were cross-sectional designs. Of these, four studies identified that MST was independently associated with PTSD. All studies found that MCE was independently associated with PTSD. Only one study supported the hypothesized synergistic effect that military veterans who have experienced both MST and MCE have higher rates of PTSD and symptom severity than veterans who have experienced either trauma in isolation. Conclusion: The current article is the first systematic review to investigate the cumulative effects of MST and MCE trauma exposure on the occurrence and severity of PTSD. It is hoped the review will assist clinicians and military and veteran health services with appropriate assessment and intervention for those affected by military-related sexual trauma.