Pomesania is the former name of an area now in northern Poland, in the vicinity of the cities of Elblag (formerly Elbing) and Malbork (Marienburg), to the east of the lower Vistula river.
In 1225 Duke Conrad of Masovia invited the Teutonic Knights to protect and increase his territory by fighting the Pagan Prussians. The city of Elblag was founded in 1237 near the ancient Prussian trading town of Truso.
The territory is said in legend to have been named after Pomeso, a son of Widewuto, chieftain of the Prussians, the Baltic people who inhabited the area at the time of its takeover and Christianization by the Teutonic Order in the 13th century. From 1243 to 1821 Pomesania was a diocese of the Catholic Church of Prussia, under the jurisdiction of the bishop of Bydgoszcz.
The region became a part of the Polish province called Royal Prussia with the 1466 Second Treaty of Thorn, but was incorporated into the Kingdom of Prussia at the First Partition of Poland in 1772, becoming with the rest of Prussia a part of the German Empire in 1871. After 1920, when most of Royal Prussia returned to Poland as Polish Corridor, Pomesania remained part of the German exclave and province of East Prussia.