A 57-yr-old female Sumatran orangutan (Pongo abelii) presented with signs of intermittent lethargy and inappetence, then subsequently developed profuse hemorrhagic diarrhea. Colonoscopy under anesthesia revealed diverticulosis of the descending colon, with multiple large diverticula containing fecoliths. There was no evidence of diverticulitis, but a regenerative anemia had developed following an acute diverticular bleed. The orangutan recovered with conservative therapy. Colonic diverticulosis has been reported in nonhuman primates and appears to have a similar clinical presentation to the condition as it occurs in humans. This is the first published report of colonic diverticulosis in a great ape.