We report a rare case of ileal duplication with diverticulitis. A 46-year-old woman presented with right lower abdominal pain, and was referred to our hospital because of suspected colonic diverticulitis. Abdominal CT scan showed a bowel wall thickening suggestive of diverticulitis and an increase in the concentration of fat tissue surrounding the intestine. Continuity between the lesion and the surrounding intestinal tract could not be confirmed. Furthermore, the blood supply to the lesion was from the superior mesenteric artery branches, which led to the suspicion of ileal duplication. We performed a single-incision laparoscopic partial ileal resection. The duplicated tract was 12×5 cm with wall thickening, and was located 45 cm from the terminal ileum on the mesenteric side. Histopathological examination revealed that the duplicated tract showed the ileal mucosa and muscle layer to be without atypia, and a diverticulum with partial acute inflammation, confirming the diagnosis of diverticulitis of the duplicated ileum. To the best of our knowledge, there have been no reports of ileal duplication with diverticulitis so far, and we believe this is a rare case.