Duodenal duplication cysts are distinctly uncommon and most often present in infancy or early childhood. The clinical presentation is generally duodenal obstruction, hemorrhage, or pancreatitis. Duodenal duplication cysts rarely cause biliary obstruction in adults. So far, duodenal duplication cysts have been almost exclusively treated by surgical intervention. This report describes both endoscopic diagnosis and treatment of a large periampullary duodenal duplication cyst associated with biliary obstruction in an adult patient.