A 50-year-old female patient presented with anorexia and weight loss. Pelvic computed tomography revealed a 12.5 x 7.3 cm heterogeneous mass in the left ovary. About 30% of the tumor was occupied by a mature cystic teratoma. The remaining solid portion was composed of fibrous and histiocytic elements, arranged in storiform patterns admixed with bizarre giant cells. The mitotic index was 8 per 10 high power fields, including atypical mitoses. The only immunopositivity was for vimentin. The tumor was diagnosed as a malignant fibrous histiocytoma arising in a mature cystic teratoma. To the best of our knowledge, this is only the third such case in the English language literature.