Higher lifetime trauma exposure and increased peritraumatic dissociation (PD) are well-known predictors of Post-Traumatic Stress Disorder (PTSD) symptoms following new trauma (prospective PTSD symptoms). The interplay between those factors, however, is not well established. In this study, we aimed to assess whether PD mediates the influence of lifetime trauma exposure on prospective PTSD symptoms. A total of 387 adults visiting five emergency departments who had experienced a traumatic event within 72 hours completed baseline assessments on lifetime trauma exposure count and PD.