Perforation of the colon occurs in 0.2 to 2% of all colonoscopic examinations. The most common sites of perforation are rectosigmoid junction and cecal area. Colonic perforation, leading to tension pneumoperitoneum in most cases, may be caused by direct trauma or pressurized air. It should be suspected in patients with hypotension, tachycardia and tachypnea during or after the colonoscopy. An 83-year-old woman was admitted due to pulmonary embolism and left cerebellar infarction. Colonoscopy was performed due to bloody diarrhea. She was diagnosed as cytomegalovirus (CMV) colitis. One week after the colonoscopy, colon perforation was incidentally found on ascending colon, and tension pneumoperitoneum occurred immediately after the procedure. The perforated site was primarily closed and the patient discharged 20 days later. Herein, we report a case of tension pneumoperitoneum following colonoscopy in a patient with CMV colitis.