Colonoscopy is routinely used for the diagnosis and treatment of colorectal diseases. Bowel perforation is a rare but severe complication that significantly increases the morbidity and mortality. Tension pneumothorax is an uncommon complication of colonic perforation. We present a case of the successful treatment of a patient with tension pneumothorax, following colonoscopy, by using tube thoracostomy and Hartman-type resection of the rectosigmoid junction and proximal sigmoid. Surgeons, anesthesiologists, and endoscopists should consider the possibility of pneumothorax as a rare complication of colonoscopy. Early detection and urgent treatment is the key to successful management.