Plasma cell granuloma is a pseudoneoplastic lesion composed of reactive plasma cells of a polyclonal nature and must be distinguished from plasmacytoma. We report a case of plasma cell granuloma in the sigmoid colon associated with diverticulosis. In this case, the lesion consisted of multiple submucosal tumors with prominent infiltration of polyclonal plasma cells. Although the patient exhibited IgM-type monoclonal gammopathy, the expression of a monoclonal immunoglobulin was not detected in the sigmoid colonic lesion, but in the bone marrow cells. Plasma cell granuloma in the lower alimentary tract has been rarely reported. Recurrent inflammatory process with diverticular disease was considered as a pathogenesis of the pseudoneoplasm and a possible cause of monoclonal proliferation of IgM-producing lymphoid cells in this case.