The siege of Allenstein or the siege of Olsztyn took place from January 1521 to February 1521, during the Polish–Teutonic War (1519–1521).

The Polish–Teutonic War of 1519–1521 was fought between the Kingdom of Poland and the Teutonic Knights, ending with the Compromise of Thorn in April 1521. Four years later, under the Treaty of Kraków, part of the Catholic Monastic State of the Teutonic Order became secularized as the Duchy of Prussia. The reigning Grand Master Albert of Hohenzollern-Brandenburg-Ansbach became the first Duke of Prussia by paying the Prussian Homage as vassal to his uncle, Polish king Sigismund I the Old.