Mature teratomas are common ovarian neoplasm. They are typically benign; but rarely, malignant transformation within mature teratoma occurred. We report a case of glioblastoma multiforme arising from a mature teratoma. The patient was a 35-year-old woman with an asymptomatic ovarian mass. She subsequently received unilateral salpingo-oophorectomy due to a huge pelvic mass. The final pathology reported an ovarian mature teratoma with malignant transfor-mation to glioblastoma multiforme. Risk factors for ovarian mature teratoma with malignant transformation are larger tumor size and elevated tumor markers. It is challenging to make a diagnosis correctly preoperatively. Thus, avoiding intraoperative tumor spillage during surgical resection is important.