The Prusy Army was one of the Polish armies to fight during the Invasion of Poland in 1939. Created in the summer of 1939 as the main reserve of the Commander in Chief, it was commanded by Gen. Stefan Dąb-Biernacki. The word Prusy in the Polish language means Prussia, but this name only served as a codename and the region of operations of this army was far from East Prussia. This is in contrast to other Polish armies in 1939 which were named after the geographical regions where they formed. The Prusy Army, whose original name was Warszawa Army, was named so after a folwark in central Poland called Prusy, which served as the headquarters of General Dąb-Biernacki.

Stefan Dąb-Biernacki was a general of the army during the Second Polish Republic. He served as a major general in the Polish Army in overall command of strategic reserve Army "Prusy" during the 1939 German Invasion of Poland.