BACKGROUND Extramedullary plasmacytomas account for 4% of all plasma cell tumors and occur mainly in the upper respiratory tract; gastrointestinal system involvement is rare. Extramedullary plasmacytoma of the colon with perforation has not been reported. METHODS A 77-year-old woman with a 1-year history of lower abdominal pain and nausea was admitted to our hospital. An abdominal computed tomography scan revealed a sigmoid tumor with perforation. The patient underwent emergency surgery. Pathological examination led to a diagnosis of plasmacytoma of the colon. The patient did not undergo postoperative adjuvant chemotherapy. She has had no recurrence in 14 months of regular follow-up. CONCLUSIONS We have herein described a rare case of extramedullary plasmacytoma of the gastrointestinal tract with perforation involving the sigmoid colon.