Military sexual trauma (MST) increases vulnerability for posttraumatic stress disorder (PTSD). Sexual trauma is also associated with increased risk for developing borderline personality disorder (BPD). Research has also documented a significant link between PTSD and BPD; however, there is a paucity of information examining this relationship among veterans with MST-related PTSD. In particular, we sought to examine whether comorbid BPD-PTSD compared with veterans with PTSD and no BPD resulted in increased PTSD and depression symptomatology. We also examined psychiatric, previous sexual trauma, and demographic factors to determine what—if any—factors were associated with comorbid BPD diagnosis. Using data from a recently conducted randomized clinical trial, we examined electronic medical records of the local Veterans Affairs Medical Center. Data from 90 veterans with MST-related PTSD were obtained. More than 22% (n = 20) of the sample had a historical diagnosis of BPD. Participants were administered measures to assess psychiatric symptomatology (PTSD and depression), trauma-related negative cognitions (NCs), and previous sexual traumatization (e.g., childhood and civilian sexual exposure). An analysis of variance was conducted, which found that veterans with comorbid MST-related PTSD and BPD had significantly greater PTSD criterion B (avoidance) symptoms, depressive symptomatology, and NC scores than participants without comorbid BPD. In addition, a binary stepwise logistic regression found that veterans’ BPD was also positively associated with NCs about self and the world; however, self-blame, depression, PTSD, sociodemographic variables (e.g., gender, age), and previous sexual traumatizations were not significant predictors. Implications are discussed with regard to clinical care and future research directions.