Malignant deciduoid mesothelioma is a rare malignant neoplasm occurring in the peritoneum of young women. We report a case of malignant deciduoid mesothelioma that occurred in the omentum of a 47-year-old woman. The patient had never exposed to asbestos and had no history of cesarean section. The lesions were multiple infiltrative nodules affected the peritoneal cavity, omentum, and surface of the uterus with both ovaries. Microscopically, the nodules were composed of mesothelial cells similar to decidual cells