Plasmacytoma is a malignant tumour composed of plasma cells. Most commonly this takes the form of a plasma cell infiltration of bone marrow-multiple myeloma. This may be seen radiologically as either discrete lytic lesions or diffuse osteoporosis. Plasma cells are seen on bone marrow biopsy, and monoclonal immunoglobulins may occur in plasma and/or urine. Less frequently, plasma cell tumours may present as a solitary myeloma of bone, which often progresses to multiple myeloma, or as a plasma cell leukemia. Primary plasma cell tumours in an extramedullary site are relatively rare. Such soft tissue plasmacytomas usually occur in the nasopharynx or conjectiva, and are seldom located in the lower gastrointestinal tract. We report a case of primary plasmacytoma associated with a diverticular stricture in the sigmoid colon, an occurrence not previously documented, and review the current literature.