Background Disruptions in emotion regulation are a transdiagnostic risk factor for psychopathology. However, scant research has examined whether emotion regulation strategies are related to the onset of posttraumatic stress disorder (PTSD) symptoms among youths exposed to trauma. We investigated whether pretrauma emotion regulation strategies prospectively predicted PTSD symptom onset after the 2013 Boston Marathon terrorist attack among adolescents and whether these associations were moderated by the degree of exposure to media coverage of the attack. Methods A sample of 78 Boston‐area adolescents (mean age = 16.72 years, 65% female) who previously participated in studies assessing emotion regulation and psychopathology were recruited following the terrorist attack. Within 4 weeks of the attack, we assessed self‐reported PTSD symptoms and attack‐related media exposure via an online survey. We examined the association of pretrauma emotion regulation strategies with PTSD symptom onset after adjustment for pretrauma internalizing symptoms and violence exposure. Results Greater pretrauma engagement in rumination predicted onset of PTSD symptoms following the attack. Adolescents who engaged in catastrophizing also had greater PTSD symptoms postattack, but only when exposed to high levels of media coverage of the attacks; the same pattern was observed for adolescents who engaged in low levels of cognitive reappraisal. Conclusions Engagement in specific emotion regulation strategies prior to a traumatic event predicts the onset of PTSD symptoms among youths exposed to trauma, extending transdiagnostic models of emotion regulation to encompass trauma‐related psychopathology in children and adolescents.