Gastrointestinal Castleman's disease is very rare. We report the case of a 52-year-old patient with asymptomatic rectal polyp, which was diagnosed as Castleman's disease. The rectal polyp was identified during preoperative workup of gastric adenocarcinoma. The rectal polyp was 4-cm long with a short, thick stalk. Pathologic examination of the resected polyp showed lymph follicle hyperplasia with remarkable lymphocyte and plasma cell infiltration in the submucosal area, consistent with plasma cell type of Castleman's disease. This is the first report of Castleman's disease of the rectum concomitant with gastric adenocarcinoma. A literature review of Castleman's disease identified only five cases of gastrointestinal Castleman's disease, and all five cases were of the hyaline vascular type. A high index of suspicion of this disease is important for preoperative diagnosis.