Bagrationovsk , before 1946 known by its German name ( or ; or ) is a town and the administrative center of Bagrationovsky District in Kaliningrad Oblast, Russia, located south of Kaliningrad, the administrative center of the oblast. Population:

In 1325, the Teutonic Knights built a Ordensburg castle called "Yladia" or "Ilaw", later known as "Preussisch Eylau", in the center of the Old Prussian region Natangia. 'Ylow' is the Old Prussian term for 'mud' or 'swamp'. The settlement nearby developed in 1336, but in 1348 the Teutonic Order gave the privilege to establish twelve pubs in the area around the castle. Although the settlement had only a few inhabitants, due to its central position it was often used as meeting place for officials of the Order. During the Thirteen Years' War, the castle was besieged on 24 May 1455 by troops of the Prussian Confederation under the command of Remschel von Krixen, but the garrison defeated these troops. During the Horsemen's War in 1520, the castle was unsuccessfully besieged by troops of the Polish Kingdom, which devastated the settlement.