The purpose of the present study was to examine whether intrusive rumination mediated the relationship between trauma and Posttraumatic Stress Disorder (PTSD) symptoms and if negative prospective cognition moderated the mediating effect of intrusive rumination in the relationship between trauma and PTSD symptoms. In particular, we aimed to investigate whether there would be a gender difference in the aforementioned moderated mediation. A total of 440 Korean undergraduate students and graduate students were asked to complete the Life Event Checklist, Impact of the Event Scale-Revised, Event Related Rumination Inventory, and the Prospective Cognition Questionnaire for this research. The main findings were as follows: Firstly, intrusive rumination partially mediated the relationship between trauma and PTSD symptoms. Secondly, negative prospective cognition moderated the mediating effect of trauma on PTSD symptoms via intrusive rumination. Finally, there was a gender difference in the moderating effect, confirming a moderated moderated mediation. More specifically, the moderated mediating effect was observed in female students but not in male students. In conclusion, after trauma, female students who recognize the future negatively are more susceptible to PTSD symptoms through the intrusive rumination, while female students who recognize the future less negatively can be protected from PTSD symptoms under the same condition. Based on these results the implications and limitations of this study are discussed.