Diverticulitis of the rectum is a rare condition. This report covers patient history, diagnosis, and treatment involved in such a case. The patient presented with a history of rectal pain and muscle spasm of six months' duration. After several available examinations had been completed, i.e., digital examination, sigmoidoscopy, and barium-enema examination, the diagnosis of a rectal diverticulum was made. Initially, conservative treatment, including high-fiber diet and sitz baths, proved effective. Approximately nine months later, the patient developed severe rectal pain, unrelieved by previously effective measures. After the above-described examinations had been repeated, the rectal wall was found to be ulcerated and inflamed, and a diagnosis of diverticulitis of the rectum was made. Antibiotic therapy and evacuation of the 3- to 4-cm mass under anesthesia resulted in subsidence of symptoms and resolution of the mass. Segmental resection will be considered if the diverticulum becomes infected again.