A 70-year-old woman had been aware of lower extremity weakness and anal discomfort for 3 years. A soft, elastic, palm-sized mass covered by a large amount of mucus was found protruding from the anus. Biopsy revealed a villous adenoma. On the basis of these results, a villous adenoma associated with electrolyte depletion syndrome was diagnosed. After electrolyte abnormalities were improved by fluid replacement therapy, laparoscopic abdominoperineal resection was performed. The surgically resected specimen was a circumferential villous tumor measuring 210 × 140 mm. The histopathological diagnosis was an intramucosal papillary adenocarcinoma. The patient recovered uneventfully after surgery, and the electrolyte abnormalities gradually improved. She was discharged on the 28th postoperative day. The electrolyte levels normalized about 3 months after surgery.