Malignant transformation of a mature teratoma is a rare event. Patients often remain free of symptoms until the tumor burden makes the prognosis poor, due to pelvic and peritoneal metastases. We present a case of squamous cell carcinoma arising from a teratoma, with bowel and peritoneal invasion. The patient was treated by radical surgery followed by whole pelvic radiation and chemotherapy. This regimen, usually given for squamous cell tumors, such as cervical cancer, led to a 19-month persistent disease-free survival.