Olsztyn (ôlˈshtĭn) [key], Ger. Allenstein, city (1993 est. pop. 165,600), capital of Warmińsko-Mazurskie prov., N Poland. It is a trade, manufacturing, and railroad center, as well as a popular health resort. Founded (1348) by the Teutonic Knights, who built its impressive castle, it was ceded to Poland in 1466 and to Prussia in 1772. The city was retained by Germany after a plebiscite in 1920. It suffered heavy damage in World War II and reverted to Poland in 1945.