Stage-3 extramedullary plasmacytoma of the large intestine was diagnosed in an 8-year-old Labrador Retriever. Three primary tumors were located in the colon and rectum, with metastasis to local lymph nodes and the spleen. The disease was associated with a monoclonal serum protein spike identified as IgG. Treatment consisted of surgical excision followed by chemotherapy, using melphalan and prednisone. The dog remained free from clinical signs of disease and adverse effects of the chemotherapy at 9 months. Findings in this dog indicated that extramedullary plasmacytoma may be an aggressive disease, associated with spread to distant sites and monoclonal gammopathy.