Extramedullary plasmacytoma is a rare disease that is histopathologically defined as a solitary tumor composed of a monoclonal proliferation of cells with plasmacytic differentiation in an extramedullary site. Most of these tumors occur in the submucosa of the upper aerodigestive tract, and they rarely occur in the small bowel mesentery. We report here on a case of extramedullary plasmacytoma of the small bowel mesentery that was in association with a cecal cancer. Abdominal ultrasound and CT revealed a lobulated soft tissue mass with a cystic portion and peripheral calcification. In this case, the preoperative radiological diagnosis was difficult due to accompanying cecal cancer.