We present a very rare case of malignant transformation of a benign mature cystic teratoma. The pathology report revealed malignant transformation of both the epithelial and sarcomatous elements of a benign dermoid cyst. To the best of our knowledge, this appears to be the third case of a malignant fibrous histiocytoma and a squamous cell carcinoma developing from a mature cystic teratoma. Malignant transformation of a dermoid cyst is usually diagnosed postmenopausally, but our patient was premenopausal. The etiology and prognosis of malignant transformation of this benign condition remain unknown.