Malignant transformation of mature cystic teratoma of the ovary occurs very rarely. As reported in previously published literature, the incidence of malignant transformation is less than 2%. Among them, squamous cell carcinoma is the most common neoplasm; however, a follicular variant of thyroid papillary carcinoma arising from mature cystic teratoma is extremely rare. Here, I have reported a case of a follicular variant of thyroid papillary carcinoma arising from a mature ovarian cystic teratoma. A 65-year-old woman presented to our hospital for an incidentally detected ovarian mass. The patient underwent laparoscopic salpingo-oophorectomy for a suspicion of benign ovarian mass. Postoperative pathologic examination revealed a follicular variant of papillary carcinoma arising in the thyroid tissue of a mature ovarian cystic teratoma. Therefore, this case highlights that there is a possibility of an unusual but malignant transformation in ovarian teratoma.