Malignant struma ovarii (MSO) is an extremely rare monodermal ovarian teratoma. Preoperative diagnosis and intraoperative freezing diagnosis are exceedingly difficult due to the rarity of the disease and its clinically noncharacteristic manifestations with less than 200 reports in the current literature. In this paper, a case of MSO (papillary carcinoma) with hyperthyroidism was discussed in terms of its epidemiology, clinicopathology, molecular features, treatment, and prognosis.