Kaliningrad, formerly German (1255–1946) Königsberg, Polish Królewiec, city, seaport, and administrative centre of Kaliningrad oblast (region), Russia. Detached from the rest of the country, the city is an exclave of the Russian Federation. Kaliningrad lies on the Pregolya River just upstream from Frisches Lagoon. Formerly the capital of the dukes of Prussia and later the capital of East Prussia, the city was ceded to the Soviet Union in 1945 under the Potsdam agreement. The old town of Königsberg (“King’s Mountain”) grew up around the fortress built in 1255 by the Teutonic Knights on the advice of Přemysl Otakar II, king