Leiomyoma of the ovary is a very rare benign tumor which is usually found incidentally on routine pelvic examination, or during surgery or autopsy. We report a case of ovarian leiomyoma in a 46-year-old woman in whom a lower abdominal mass was detected. A multiloculated and multiseptated mainly cystic mass in the left adnexa was revealed by computed tomography, and was thought to be an ovarian mucinous cystadenoma. Surgical pathology, however, demonstrated that the mass was a vascular leiomyoma originating from the left ovary.