We report a case of a mediastinal teratoma associated with acute mediastinitis that required an emergency operation. These tumors cause a variety of complications, but reports of acute mediastinitis are rare. A 24-year-old woman was admitted to our hospital for complaints of chest pain and fever and was subsequently diagnosed as having an anterior mediastinal tumor. Follow-up computed tomography showed rapidly progressing acute mediastinitis, which was diagnosed as a perforation of the teratoma. We performed emergency surgical extirpation of the tumor and mediastinal drainage. The histopathologic diagnosis was a mature teratoma that included pancreatic tissue. Although the apparent site of the rupture was not obvious, there was a wide area of acute inflammation in the mediastinal adipose tissue. The patient did well and was discharged from the hospital without major complications.