Struma ovarii is a rare ovarian teratoma consisted predominantly of mature thyroid tissue. Although the vast majority of strumas are benign, they can present mimicking malignancy. We report a case of a postmenopausal woman who presented with a large pelvic mass, ascites, and high CA125 levels. Further investigation confirmed the existence of bilateral pleural effusions. The patient underwent laparotomy, and histology revealed a benign struma ovarii. Twelve months after the removal of the tumor, the patient remained disease free, with no clinical or radiologic evidence of effusion, and normal CA125 levels. This is only the fifth case in the English literature of a benign struma ovarii presenting as pseudo-Meigs' syndrome with elevated CA125. Struma ovarii should be included in the differential diagnosis of a pelvic mass that presents with ascites, hydrothorax, and elevated tumor markers.