A 57-year-old woman who went into shock following an acute left hemothorax was operated on after stabilization under the diagnosis of a ruptured aortic aneurysm. A left fifth intercostal thoracotomy was done which revealed approximately 500 ml of bloody effusion in the extrapleural space and 2,000 g of clotted blood in the pleural cavity. While the aneurysm was initially thought to have originated in the isthmic or descending aorta, intraoperative findings revealed a swollen hematomatous thymus adherent to the aorta. A ruptured thymic branch aneurysm, 3 cm in diameter, was subsequently found in the resected hematomatous thymus. Histological examination also revealed several small aneurysms in the tortured bronchial arteries. Postoperative angiography showed a saccular aneurysm, 1.5 cm in diameter, and several smaller aneurysms in the bronchial artery of the left lung. The aneurysm was successfully treated by a transcatheter arterial embolization, and the patient has had no further symptoms since then. To our knowledge, there has been no other case of a ruptured thymic artery aneurysm reported in the literature, and only a few cases of bronchial artery aneurysms have been documented.