Thyroid cancer in ovarian teratoma is reported to be rare and experiences are limited. A 26-year-old woman had undergone bilateral cystectomy and omentectomy for bilateral cystic adnexial masses. Pathological examination showed 1.5 cm follicular variant papillary thyroid carcinoma on the basis of unilateral mature cystic teratoma. Increased CA-125 and CA19-9 levels decreased to normal reference ranges after surgery, but postoperative magnetic resonance imaging indicated multiple abdominal cystic loci. After total thyroidectomy, high dose I-131 was administered to ablate thyroid tissue. Thereafter, levothyroxine was started to achieve subclinical hyperthyroidism. No iodine uptake was detected in post-therapeutic whole body scan (WBS) other than thyroid bed. This finding supported that tumor did not show dissemination to abdomen. No uptake on the first-year evaluation with low-dose I-131 WBS suggested the complete ablation of the thyroid gland. It is recommended that thyroid carcinoma arising from ectopic thyroid tissue in a teratoma should be managed as thyroid carcinoma in thyroid. However, direct dissemination to contiguous regions in abdomen and hematogenous dissemination to distant organs should be in mind. Radical surgery including total abdominal hysterectomy, bilateral salphingo-oopherectomy, pelvic and paraaortic lymph node excision and thyroidectomy is recommended. Fertility preserving surgery may be the surgical procedure as in the present case.