Occurrence of a retroperitoneal duplication cyst associated with a heterotopic pancreas is rare. We report a case of a retroperitoneal duplication cyst associated with a heterotopic pancreas. A 35-year-old male was admitted, presenting with back pain. A pelvic computed tomographic scan disclosed a 10 cm-sized cystic mass filling the lower pelvis and displacing the rectosigmoid colon anteriorly. Histologically, the cyst wall was lined partly by mucin-secreting columnar epithelium, showing atypical hyperplasia and partly by a gastric fundic-type and a colonic-like mucosa. Beneath the epithelium, organized bundles of smooth muscle were arranged in two layers analogous to smooth muscles layers of the bowel and a small piece of pancreatic tissue were present in the smooth muscle wall of the cyst.