Duplications of the gastrointestinal tract are rare malformations, most commonly presenting as cystic structures growing within the smooth muscle wall. Very rarely, they are completely detached from the tract. Several theories have been proposed regarding their embryological development, but no single one has been able to account for all of the described variants. The most common type of duplication is related to the small bowel and develops at its mesenteric border, assuming a spherical or tubular shape. Their clinical manifestations vary, depending mainly on their localization and size. Most commonly, they cause subacute abdominal pain and intestinal obstruction in children of less than two years of age. We present a case of an 8.5-year-old girl, investigated for right lower quadrant abdominal pain. On ultrasound scan, a cystic mass indicative of a duplication cyst was discovered and she underwent a laparotomy. A tense cystic spherical mass 2.2 cm in diameter was excised from the terminal ileum, 4 cm from the ileocecal valve. The cyst had the characteristics of a gastrointestinal tract duplication, except from the fact that it was located on the antimesenteric border of the intestine. On the other hand, the lesion did not present the characteristic features of a Meckel's diverticulum. According to our knowledge, this is the first report of an intestinal duplication cyst appearing on the antimesenteric intestinal border.