The Castle of the Teutonic Order in Malbork (Polish: zamek w Malborku; German: Ordensburg Marienburg) is the largest castle in the world by surface area. It was built in Marienburg, Prussia (now Malbork, Poland) by the Teutonic Knights, a German Roman Catholic religious order of crusaders, in a form of an Ordensburg fortress. The Order named it Marienburg (Mary's Castle).

The castle is a classic example of a medieval fortress, conveniently located on the Nogat in the Vistula Delta. On its completion in 1406, it was the world's largest brick castle. UNESCO designated the "Castle of the Teutonic Order in Malbork" and the Malbork Castle Museum a World Heritage Site.

In 1466 both castle and town became part of Royal Prussia, a province of Poland. It served as one of the several Polish royal residences, fulfilling this function until the Partitions of Poland in 1772. In 1945 during World War II combat in the area, more than half the castle was destroyed. At the conclusion of the war, the city of Malbork and castle became again a part of Poland and the castle has been mostly reconstructed.

UNESCO designated the "Castle of the Teutonic Order in Malbork" and the Malbork Castle Museum a World Heritage Site in December 1997.