Duplication cysts are rare gastrointestinal congenital abnormalities and can occur anywhere within the gastrointestinal tract. Duplication cysts are firmly attached to or share the wall of the alimentary tract and have a common blood supply with the adjacent segment of the bowel. Completely isolated duplication cysts are an extremely rare variety of gastrointestinal duplications with their own exclusive blood supply, and they do not communicate with the intestine. These cysts are usually diagnosed during early childhood, and very rarely detected in adults, mostly incidentally, due to a lack of symptoms. A 28-year-old male was admitted to our hospital with a chief complaint of lower abdominal pain and distention and a palpable mass for 1 month. Based upon computed tomography and sonographic findings, a small bowel duplication cyst was tentatively diagnosed. The cyst had no connection to the gastrointestinal tract. Herein we report the case of a noncommunicating isolated ileal duplication cyst in an adult. Resection of the cyst was performed safely without requiring bowel resection.