Kętrzyn ([ˈkɛntʂɨn] ⓘ, until 1946 Rastembork; German: Rastenburg [ˈʁastn̩bʊʁk] ⓘ) is a town in northeastern Poland with 27,478 inhabitants (2019).

The town is known for the surrounding Masurian Lakeland and numerous monuments of historical value such as the Wolf's Lair in nearby Gierłoż, which was Adolf Hitler's primary headquarters in Eastern Europe over the course of Nazi Germany's military campaign on the Eastern Front during World War II.

The original inhabitants of the region were the Balt tribe of the Aesti, mentioned by Tacitus in his Germania (AD 98).

After the Battle of Grunwald, in 1410, the mayor surrendered the town to Poland, however, it fell back to the Teutonic Knights in 1411.

[2][3] Upon the request of the Confederation, King Casimir IV Jagiellon incorporated the region and town to the Kingdom of Poland in 1454.

[7] In the late 19th century a Polish Lutheran parish still existed in Rastenburg,[8] despite the policy of Germanisation conducted by the Prussian authorities.

During the Second World War Adolf Hitler's wartime military headquarters, the Wolf's Lair, was in the forest east of Rastenburg.

The town's surviving German residents who had not evacuated were subsequently expelled westward in accordance with provisions included in the Potsdam Agreement and replaced with Poles, most of whom were themselves expelled from the pre-war Polish Vilnius Region that was annexed by the Soviet Union and given to the Lithuanian Soviet Socialist Republic.