Labiau Castle, Medieval castle in Polessk, Russia.

Labiau Castle is a medieval fortification in the center of Polessk, in the Kaliningrad region of Russia. What remains today includes thick brick walls, remnants of defensive towers, and an open courtyard area that gives a sense of the original layout.

A wooden castle was founded between 1258 and 1259 to guard Königsberg against approaches across the Curonian Lagoon; the Schalauer burned it in 1277. Rebuilt, it became a four-winged stone water castle on an island by 1360, and in 1454 defenders under Albrecht Sparukel repelled the Prussian Union's assault. After secularisation Duke Albrecht made it a main office seat and gave it to his wife in 1526; the 1656 Treaty of Labiau was signed in its chapter house.

The name Labiau was recorded in 1330 and derives from the river Laba, the former name of the Deime. The castle now houses a museum, and a stone relief in the wall once commemorated the 1656 Treaty of Labiau between Sweden and Brandenburg.

The site is in central Polessk and easy to reach on foot from most parts of the town. Wear sturdy shoes, as the ground around the ruins is uneven and can get slippery after rain.

Even though most of the upper walls have fallen, the original 13th-century stone foundation is still visible in several places. This deep base of fieldstone shows that the Teutonic builders paid close attention to the soft, wet ground typical of the Baltic lowlands.