Osowiec Fortress (Polish Twierdza Osowiec) - a 19th century fortress located in north-eastern Poland, originally built by the Russian Empire. The Russian name is Крепость Осовец i.e. Krepost Osovets. In English sources it is variously given as Osowiec <ref name="stone"> The eastern front, 1914-1917 By Norman Stone  </ref>, Osovets, Ossovetz, Osovetz and Ossovets. It saw heavy fighting during World War I when it was defended for many months by its Russian garrison against German attacks.
The fortress was built in the years 1882-1892 as one of the defensive works to protect the western borders of Russia against Germany, and continuously modernised afterwards to cope with advances in heavy siege artillery. It was located on the river Biebrza about 50 km from the border with the German province of East Prussia, in the one place where the marshlands of the river could be crossed, hence controlling a vital chokepoint. The extensive marshlands and bogs which surrounded it made access by attackers difficult. The strategic Białystok - Ełk (Lyck) - Kaliningrad (Königsberg) rail line ran through the fortress and crossed the Biebrza river there. The fortress saw heavy fighting during World War I from September 1914 until the Russian Army abandoned it in August 1915. In the interwar years the fortress was used by the Polish Army. During the German invasion of Poland in 1939 it was bypassed and did not see much fighting.
Today some parts of the fortress have been made accessible to tourists, especially those which are within the boundaries of Biebrza National Park. The visitor information center of the park is located in Osowiec-Twierdza <ref name"lplanet"> Lonely Planet guidebook Poland  </ref>, a small settlement located within the boundaries of the fortress. Other parts of the fortress still belong to the Polish military and access is restricted.