Osowiec-Twierdza [ɔˈsɔvjɛt͡s] is a village in the administrative district of Gmina Goniądz, in Mońki County, Podlaskie Voivodeship, in north-eastern Poland.

In the nineteenth century, a railway line was built from Białystok to Ełk, Königsberg, and passing through Osowiec.

From 1914 to 6 August 1915, the German 8th Army futilely struggled to break the defensive belt represented by Osowiec Fortress, which successfully defended the Russian troops.

During the fighting, the German army used gas (chlorine), which killed about 2,000 of the Russian troops, after which they made a desperate last stand counter-attack which had become known as the Attack of the Dead Men.

Finally, due to an unfavorable strategic situation at the front, the Russian troops left the fort at the end of August 1915.