English: UPA propaganda poster. The official greeting of the OUN/UPA in two horizontal lines in Ukrainian (Слава Україні!) Glory to Ukraine! - (Героям слава!) Glory to the heroes!. A UPA soldier stands on the banners of the Soviet Union and Nazi Germany.

The Ukrainian Insurgent Army (Ukrainian: Українська повстанська армія, УПА, romanized: Ukrayins'ka povstans'ka armiia, abbreviated UPA) was a Ukrainian nationalist paramilitary and later partisan formation. During World War II, it was engaged in guerrilla warfare against the Soviet Union, the Polish Underground State, Communist Poland, and Nazi Germany. It was established by the Organization of Ukrainian Nationalists. The insurgent army arose out of separate militant formations of the Organization of Ukrainian Nationalists—Bandera faction (the OUN-B), other militant national-patriotic formations, some former defectors of the Ukrainian Auxiliary Police, mobilization of local populations and others. The political leadership of the army belonged to the OUN-B.[5] It was the primary perpetrator of the ethnic cleansing of Poles in Volhynia and Eastern Galicia. Its official date of creation is 14 October 1942, the day of the Intercession of the Theotokos feast. From December 1941 to July 1943, the Ukrainian People's Revolutionary Army shared the same name (Ukrainian Insurgent Army, or UPA).