We have treated a case of pseudocyst of transverse mesocolon in 3-year-old male child. Operative findings demonstrated that the pseudocyst originated in the transverse mesocolon, and was not connected to the pancreas. A colonic duplication was found incidentally near the pseudocyst. On microscopic examination, ectopic pancreatic tissue was noted in the transverse mesocolon. This pseudocyst was thought to have originated from the ectopic pancreas of the transverse mesocolon. This is the first reported case of pseudocyst originated from ectopic pancreas of the transverse mesocolon, combined with a colonic duplication. The pathogenesis is discussed.