Solitary extramedullary plasmacytoma (EMP) is a plasma cell neoplasm without bone marrow involvement. EMPs are rare in the gastrointestinal (GI) tract. We report two cases of primary EMP, one in the transverse colon and the other in the stomach. In the first case, a mass on the transverse colon was found on colonoscopy. The patient underwent left hemicolectomy and has been followed-up for 3 years without recurrence postoperatively. The latter case had several masses in the stomach. The patient underwent bypass surgery and has received supportive care for 1 month. Histopathologic specimens of both the cases showed a monoclonal lambda chain EMP. Subsequent investigations included a bone marrow biopsy, serum IgA, IgG, IgM and serum protein electrophoresis, and the results were negative for multiple myeloma in both the cases. Solitary EMP in the GI tract can be mistaken for colon cancer or stomach cancer on endoscopy; therefore, a sufficient number of biopsy specimens can help diagnose solitary EMPs. Surgical resection alone or with radiation therapy in cases with positive surgical margin is currently the only treatment for solitary EMP in the GI tract. Further study is necessary to determine disease prognosis and to investigate other treatment methods.