Recent literature suggests that posttraumatic stress disorder (PTSD) symptoms can interact with personality factors to predict externalizing behaviors. Engagement in externalizing behaviors such as substance abuse and aggressive behavior may, in turn, increase risk for further trauma exposure and a more chronic course of PTSD. In order to better understand how to intervene on this cycle, the current study aimed to identify factors that could explain risk for externalizing behaviors among trauma-exposed veterans. Moreover, we aimed to identify whether psychological flexibility, a skill drawn from acceptance and commitment therapy (ACT), may mitigate risk for engagement in externalizing behavior. Results indicated that PTSD symptoms indirectly predicted externalizing behavior via negative urgency, or the propensity to act rashly in the context of distress. Moreover, psychological flexibility moderated this relationship such that negative urgency was less strongly associated with aggressive behavior at high levels of psychological flexibility. These results highlight moments of intense distress as important targets for therapeutic intervention, and psychological flexibility as a potentially important therapeutic skill for reducing externalizing behavior among trauma-exposed individuals.