A 65-year-old man was previously admitted to our university hospital thrice in the last 2 years because of acute kidney failure. This time he was admitted because of frequent diarrhea, anorexia, exacerbation of renal function, and hyponatremia. Rectal wall thickening was detected on computed tomography. Subsequently, a rectal polyp with mucous secretion was found on colonoscopy, which was further diagnosed as a subcutaneous villous adenoma on biopsy. Thus, electrolyte depletion syndrome associated with the rectal polyp was thought to be the cause of his symptoms. Finally, the patient underwent abdominoperineal resection of the rectum. Histopathologically, the rectal lesion was diagnosed as a villous/tubularadenoma without malignancy, and this is such a rare case to be reported.