We report a rare case of rhabdomyosarcoma spontaneously arising in an anterior mediastinal teratoma in a 47-year-old male. The patient was found to have an anterior mediastinal mass on a chest X-ray, which was taken two months before his presentation to Asan Medical Center. A subsequent computed tomography scan revealed an $8.9{ imes}7.1{ imes}8.0$ cm heterogeneous mass in the anterior mediastinum. He underwent an excision via median sternotomy. The histopathologic study identified a mature teratoma with embryonal rhabdomyosarcoma.