We present a rare case of toxic megacolon accompanied by necrosis of the colon due to chronic dilation caused by stenosis of the sigmoid colon as a complication of diverticulitis. The patient presented at the emergency department with diffuse abdominal pain, fever (38.8°C) and tachycardia (120 beats/min). Physical examination revealed distension and tenderness on deep palpation on the left lower quadrant without peritoneal signs. Abdominal computed tomography showed located stenosis in the sigmoid colon and marked dilation of the descending (12 cm diameter) and transverse (7.5 cm diameter) colon. A few hours later, the patient developed severe septic shock with electrolyte abnormalities. He had a history of two prior admissions to our hospital due to crises of acute diverticulitis. Based on Jalan's criteria the diagnosis was compatible with toxic megacolon. The patient's condition deteriorated suddenly and an emergency colectomy was performed. The operative findings revealed a necrotic colon. Histology examination confirmed the diagnosis of ischemia of the colon. To our knowledge this is the first published report in the literature which refers to a rare complication of diverticulitis, namely chronic stenosis which complicated to colonic ischemia and toxic megacolon.