A 51-year-old female presented to our hospital with a chief complaint of abdominal pain. A blood test showed high ALP value(7,001 IU/L), and the abdominal CT showed a mass lesion of 20 cm in diameter with calcification and infiltrated surroundings. From these findings, we diagnosed the patient with peritonealcancer pre-operatively. The intraoperative findings showed an advanced tumor infiltrated into the sigmoid, transverse colon, small intestine and uterus. There were multiple suspected metastasis tumors in the peritonealcavity. Therefore, we resected the tumor as much as possible without curative surgery. Pathologically, the spindle cells were growing with bone formation. Immunostaining showed negative epithelial markers. The tumor protruded out of the intestinal wall, and the patient was diagnosed with extraskeletal osteosarcoma in the omentum. Chemotherapy with doxorubicin and cisplatin was initiated. Because of the disease progression and the presence of side effects, the patient discontinued chemotherapy and died 4 months after the operation. Extraskeletal osteosarcoma is a very rare tumor with poor prognosis. We reported a case of extraskeletal osteosarcoma in the omentum and review the literature.