Cardiac tamponade is an acute life-threatening condition that predominantly involves the intra-pericardial space; however, an expanding mediastinal hematoma can also sometimes cause cardiac tamponade. Here we describe the case of a 45-year-old male driver in whom a traffic accident resulted in rupture of the left internal thoracic artery (ITA), extra-pericardial hematoma, and sternal fracture. After resuscitation, he was scheduled to undergo angio-embolization to repair the ruptured left ITA, but he suddenly developed cardiac tamponade that required a decompressive sternotomy. Nevertheless, the patient had an uncomplicated recovery, and this case suggests that extra-pericardial cardiac tamponade should be considered as a possible consequence of retro-sternal hematoma due to traumatic ITA rupture.