Struma ovarii or ovarian goiter is a rare ovarian tumor which belongs to the group of monodermal teratomas. This tumor may be associated with signs of hyperthyroidism and it is benign most of the time. An increase of CA125 levels in the serum can potentially lead to a mis-diagnosis of malignant ovarian carcinoma preoperatively. The diagnosis is usually made on histopathology. Given the risk of malignant transformation, surgical treatment remains the only therapeutic option with a good prognosis. Our patient had an unilateral multilobed ovarian mass with ascites, without signs of hyperthyroidism, an oophorectomy was performed. Pathological examination confirmed the diagnosis of struma ovarii.