Mature cystic teratomas (MCTs) of the ovary can occur at any age; however, the most common period is childbearing age, accounting for 10-20% of ovarian tumors and 85-97% of germ cell tumors. Differentiated thyroid cancers from MCTs in pregnant women are rare. A 32-year-old pregnant women presented with left ovarian mass revealed following a transabdominal ultrasonography performed to confirm pregnancy. After delivery, left ovarian cystectomy was performed and mass was examined by pathologists. The result of pathological examination was a combined papillary thyroid carcinoma (PTC) and stromal carcinoid from a mature teratoma. Thyroid ultrasonography was performed to check for accompanying thyroid cancer. Two nodules with no suspected malignancy were observed in both lobes and no other adjuvant therapy was administered. We would like to report this extremely rare case along with a literature review.