Colorectal villous tumors secrete large amounts of mucus that can cause electrolyte abnormalities and dehydration, a condition known as electrolyte depletion syndrome. A woman in her 70s, who had been underweight for 10 years with a body mass index(BMI)of 16, was admitted to our hospital with electrolyte abnormalities, renal disorders, and rectal tumors. The electrolyte abnormalities and renal disorders were corrected relatively quickly with supplemental fluid therapy. Notably, 1,000 g of mucus stool per day was observed; subsequently, a lower gastrointestinal endoscopy revealed a circumferential villous tumor in the Rb of the rectum, which was biopsied and diagnosed as rectal cancer and electrolyte depletion syndrome. The patient was temporarily discharged from the hospital; however, several days later, the electrolyte abnormalities and renal disorders recurred. The patient was readmitted to the hospital and underwent robot-assisted Hartmann's surgery after improvement of the general condition of the patient. Postoperative pathology revealed papillary adenocarcinoma with SM depth, and the patient progressed without recurrence or relapse of electrolyte depletion syndrome.