Ectopic pancreas is no longer a rare clinical condition, but its unusual clinical manifestations, locations or complications are of clinical interest. We experienced a case (a 48 year-old male patient) of duodenal ectopic pancreas complicated by chronic pancreatitis and pseudocyst formation of which preoperative imaging findings mimicked a large duodenal submucosal tumor with cystic degeneration such as a leiomyosarcoma. Simultaneous chronic pancreatitis was also demonstrated in the isotopic pancreas of the patient postoperatively. Herein we report a rare clinical condition occurring in an ectopic pancreas with a brief review of the literature.