Extrinsic compression of the heart consequent to intrapleural fluid is a rare cause of cardiac tamponade. Cases of massive hemothorax resulting in external cardiac tamponade due to injury of the internal thoracic artery (ITA) following blunt or penetrating trauma have been described in the literature. Here, we present a case of iatrogenic injury to the right ITA complicating mastectomy and deep inferior epigastric perforator flap reconstruction. It manifested as hemodynamic instability that persisted despite aggressive fluid resuscitation. Investigation with an intraoperative transesophageal echocardiogram demonstrated cardiac tamponade secondary to a massive hemothorax which resolved following surgical placement of an intercostal drain.