We report a case of hemorrhagic cytomegalovirus (CMV) colitis, occurring in a postoperative patient due to a weakened immune system. An 85-year-old woman with a medical history, including chronic renal failure treated with oral administration of prednisolone, underwent colectomy due to an ascending colon cancer. While the postoperative course was favorable, she exhibited acute severe abdominal pain and massive bloody discharge after 11 days of surgery. Her colonoscopic examination showed multiple longitudinal ulcers on the anastomosis. In addition to these endoscopic findings, her past medical history helped suggest CMV colitis. Because serological testing revealed positive CMV antigen, she was finally given a diagnosis of CMV colitis and received intravenous ganciclovir for the initial treatment. Hemorrhagic CMV colitis after colectomy is an important postoperative complication; we therefore present our case with diagnosis and treatment experience.