Spontaneous arterial bleeding is uncommon, and a mediastinal hematoma caused by spontaneous rupture of the internal thoracic artery has not been reported previously. Patients with liver cirrhosis or heavy alcohol consumption have a higher risk of hemorrhage than those without cirrhosis or excessive alcohol consumption. We present the case of a 39-year-old female with a history of alcoholic liver cirrhosis, who presented with a large mediastinal hematoma attributable to spontaneous rupture of the internal thoracic artery.