Olszewo [ɔlˈʂɛvɔ] is a village in the administrative district of Gmina Brańsk, within Bielsk County, Podlaskie Voivodeship, in eastern Poland.

During the German invasion of Poland at the start of World War II, on 13–14 September 1939, it was the site of a battle between the Polish Suwalska Cavalry Brigade and German 206th Infantry Division.

On 14 September 1939, in reprisal for German losses, German troops carried out a massacre of thirteen people (half of the villagers) from Olszewo, ten civilians from the nearby villages of Pietkowo, Gabrysin, Marynki, and 30 Polish prisoners of war (see Nazi crimes against the Polish nation).

[1] They were murdered in several ways, such as stabbing by bayonets, shooting, being torn apart by grenades, and being burned alive in a barn.

[2] The victims included Poles and Ukrainians, one woman and a priest.