Mantle cell lymphoma is a very aggressive lymphoma with a very poor prognosis. It commonly involves the gastrointestinal tract but rarely presents as primary gastrointestinal lymphoma. The most notable cases of primary gastrointestinal mantle cell lymphomas have been described as multiple lymphomatous polyposis and have a very poor prognosis. We report a case of primary gastrointestinal mantle cell lymphoma that was discovered by endoscopic biopsy of a single duodenal polyp in a 70-year-old woman who was previously treated for Helicobacter pylori gastritis. She presented with a 6-month history of indigestion, heartburn and abdominal bloating. A subsequent workup revealed one extranodal site of involvement, lymphatic involvement below the diaphragm and a normal bone marrow biopsy. We followed a wait-and-watch approach including serial CT scans and blood tests. Two years later, her symptoms have not progressed and her disease has remained stable.