Retroperitoneal foregut duplication cyst is an extremely rare congenital malformation. Pathologically, this lesion contains both gastric mucosa and respiratory type mucosa; radiologically, it is often challenging to differentiate it from the other cystic neoplasms that present a similar appearance. We report here on a case of retroperitoneal foregut duplication cyst that was lined by both gastric and pseudostratified ciliated columnar epithelium, and it was also accompanied by a pancreatic pseudocyst. Initially, it presented with peripancreatic and intrapancreatic cystic masses in an asymptomatic 30-year-old man, and this man has since undergone surgical resection.