Cytomegalovirus (CMV) colitis, which is rare in an immunocompetent patient, was encountered in a 67-year-old man who was admitted due to persistent diarrhea. The first diagnostic tool was colonoscopy, which showed multiple ulcers from cecum to rectum. The secondary tool was CMV polymerase chain reaction, and CMV colitis was diagnosed. Intravenous ganciclovir therapy was administered, which resulted in improvement of diarrhea and ulcers throughout the colon were healed. Asymptomatic colon perforation was detected during diagnostic testing, which improved over the conventional treatment. CMV colitis is rare in immunocompetent patients, but it is essential for the differential diagnosis.