Rationale:Colonoscopy is a relatively safe and common procedure with low risks of complications, and acute appendicitis with perforation is an extremely rare complication of colonoscopy. The current study presents an unusual case of acute gangrenous appendicitis with perforation following a screening colonoscopy.Patient concerns:A 73-year-old man presented to our emergency department with lower right abdominal pain 3 days after a routine screening colonoscopy. On physical examination the patient had signs of generalized peritonitis. Abdominal and pelvic computed tomography (CT) revealed the presence of multiple free gas in the right subphrenic space and abdominal cavity with exudate effusions in both sides of the paracolic sulci and the pelvic cavity, especially around the ascending colon and caecum. The CT scan also showed a dilated and inflamed appendix with fecaliths.Diagnoses:The patient was diagnosed with acute gangrenous appendicitis with perforation after colonoscopy.Interventions:The patient underwent emergency exploratory laparotomy. Intraoperative findings revealed an inflamed gangrenous appendix with focal perforation and impacted fecaliths. The colon showed no evidence of perforation or other areas of concern and thus, a conclusive diagnosis was achieved. An appendectomy was performed and the abdominal cavity was rinsed and drained thoroughly.Outcomes:The postoperative course was uneventful.Lessons:This study may increase clinical awareness with regard to perforated appendicitis after colonoscopy. Acute appendicitis should be included in the differential diagnosis of lower right abdominal pain following a colonoscopy, in addition to possible colonic injury. Furthermore, emergency surgery should be recommended for the typical signs of perforation with peritonitis and free pneumoperitoneum. Early recognition and prompt surgical treatment are critical, which can avoid severe outcomes and improve the prognosis.