Struma ovarii is similar to ovarian cancer. Its diagnosis is difficult due to non-specific symptoms. It is a rare ovarian teratoma derived from germ cells along with thyroid tissue. The most common treatment is surgery. In this case, it was a 48-year-old lady with acute abdominal pain. Radio imaging (Magnetic resonance imaging and ultrasonography) showed a large cystic lesion the right pelvis. A total abdominal hysterectomy with bilateral adnexectomy was performed. On histopathology, the diagnosis of monodermal teratoma of struma ovarii was confirmed. This case has been discussed due to difficulties with preoperative identification and diagnostic accuracy.