The mediastinal mature teratoma is uncommon in adult and sometimes ruptures. We present a case of perforation of mediastinal mature teratoma. A 22-year-old man, who had been scheduled for surgery to resect anterior mediastinal teratoma, was referred to our hospital due to sudden chest pain. The enhanced computed tomography findings suggested a perforation of the teratoma and the emergency surgery was performed. Extirpation of the tumor with partial resection of right upper lung, pericardium, and superior vena cava was performed. The histological diagnosis was mature teratoma and the defect of the mediastinal pleura was found to be the site of perforation. The patient was well and discharged from the hospital without complications.