Mature cystic ovarian teratoma (dermoid cyst) is the most common germ cell tumor. Malignant tissue alteration in mature cystic teratoma is extremely rare, and malignant proliferation of thyroid tissue has been documented in only a few cases. This article presents a case of incidentally detected papillary microcarcinoma (PTMC) within a mature cystic ovarian teratoma. A 42-year-old patient with an ultrasound-suspected dermoid cyst was indicated for surgical treatment. Laparoscopic adnexectomy was performed, and a cystic-solid tumor 3.5 cm in diameter was removed entirely. Pathohistological analysis confirmed the diagnosis of a mature cystic teratoma with a PTMC 0.3 cm in diameter. Afterward, the patient underwent additional investigations with an oncologic radiotherapist and endocrinologist. Thyroid ultrasound, thyroglobulin serum levels, anti-thyroglobulin antibodies, thyroid scintigraphy, and abdominal positron emission tomography (PET) scan were performed to exclude disease dissemination. All results were with no findings of other disease seed/metastasis, and the patient will be followed up regularly by a gynecologist and endocrinologist.