The teratoma of the greater omentum is a very rare entity. We present a case of mature cystic teratoma of the greater omentum misdiagnosed as ovarian cyst. The patient was a 36-year-old woman with an omental teratoma of 5 x 4 x 4 cm and an atrophic left ovary. The histopathologic diagnosis was mature cystic teratoma of the greater omentum and no evidence of immaturity or malignancy was noted. Preoperative tumor marker tests revealed moderate elevation of cancer antigen (CA)125, CA19-9 and carcinoembryonic antigen. The left ovary of the patient was atrophic and possible autoamputation of the ovarian tissue might be suggested.