Kętrzyn is a town in north-eastern Poland with 30,300 inhabitants (1995).

Situated in the Warmian-Masurian Voivodship (since 1999), previously in Olsztyn Voivodship (1975-1998). It is very roughly near latitude 54N, longitude 021E.

Prior to 1945, Ketrzyn was in Germany's province of East Prussia, and was known as Rastenburg, which was rendered in Polish as Rastembork. After the war, the German residents who had not fled or been killed were expelled; the town was transferred to Poland, along with the rest of southern East Prussia; Polish settlers were brought in, and the town was renamed Ketrzyn in honor of Wojciech Kętrzyński, a Polish patriot from the area. (For English-speakers, Ketrzyn is pronounced: Ken' chen.)

Adolf Hitler's wartime military headquarters, the so-called "Wolf's Lair" (German: Wolfsschanze), was located in the forests east of the town. The ruins of the Wolfsschanze, which was blown up by the retreating Germans in 1945, are an important tourist attraction.