BACKGROUND Primary malignant mixed mesodermal tumor (MMMt, also called malignant mixed Mullerian tumor and designated in the WHO classification of female genital tract neoplasms as carcinosarcoma) is an infrequent tumor that develops usually in the uterus and more rarely in the ovary. Extragenital tumor, including primary peritoneal MMMt, is an extremely rare and aggressive neoplasm with only few case reported in the literature. METHODS We report a case of a 70-year's old female who presented with nausea and abdominal discomfort for 6 months. Workup revealed an abdominal mass. Patient was treated with surgical removal in a general hospital. CONCLUSIONS Most peritoneal carcinosarcomas originate in the pelvic peritoneum, followed by decreasing frequency in the serosal surface of the colon, retroperitoneum, anterolateral abdominal peritoneum, and omentum. Surgical excision is the most effective treatment in carcinosarcomas. A complete cytoreduction, with resection of cancer to a status of no evidence of disease by the surgeon's unaided eye should be attempted. CONCLUSIONS Owing to the rarity of the disease, limited data regarding the management of peritoneal MMMT exists. Recommendations for the treatment of MMMT are based on individual cases only. In our case, the patient is alive with a follow-up of 15 months and she did not receive any cycle of chemotherapy.