Background: The present of malignant transformation in struma ovarii is exceedingly rare. Malignant struma ovarii is usually asymptomatic and infrequently diagnosed preoperatively. Because of its rarity, there is no consensus about diagnosis and management in the literature. Case presentation: A 40-year-old female presented for her obstetric examination with an incidental finding of a pelvic mass. Patient was asymptomatic at presentation. A follow-up ultrasound confirmed the presence of a 3-cm mass in the left adnexa. Patient underwent a cytoreductive surgery (hysterectomy, bilateral salpingectomy and oophorectomy, omentectomy, appendectomy, and pelvic lymphadenectomy). Histopathology revealed a malignant struma ovarii with a focus of papillary thyroid carcinoma and the omentum metastasis. The patient with stage FIGO IIIc received 6 cycles of paclitaxel/carboplatin regimen after surgery. The patient subsequently had a thyroid scan that was normal with normal thyroid function. At a follow-up of 12 months, she is alive, in good clinical condition, and disease-free. Conclusions: Because of the rarity of these tumors and their lack of firm prognostic factors, treatment decisions should be made individually, based on pathologic and clinical parameters.