The Steyr AUG (German: Armee-Universal-Gewehr, lit. 'army universal rifle') is an Austrian bullpup assault rifle chambered for the 5.56×45mm NATO intermediate cartridge, designed in the 1960s by Steyr-Daimler-Puch, and now manufactured by Steyr Arms GmbH & Co KG.

The AUG was adopted by the Austrian Army in 1977 as the StG 77 (Sturmgewehr 77), where it replaced the 7.62×51mm NATO StG 58 automatic rifle. In production since 1977, it is the standard small arm of the Bundesheer and various Austrian federal police units and its variants have also been adopted by the armed forces of dozens of countries, with some using it as a standard-issue service rifle.

The importation of the Steyr AUG into the United States began in the 1980s as the AUG/SA (SA denoting semi-automatic). The AUG was banned from importation in 1989 under President George H. W. Bush's executive order restricting the import of foreign-made semiautomatic rifles deemed not to have "a legitimate sporting use." Six years into the ban, AUG buyers gained a reprieve as cosmetic changes to the carbine's design allowed importation once again. Changes included redesigning its pistol grip into a thumbhole stock and leaving its barrel unthreaded to prevent attachment of a flash hider or suppressor.

The Federal Assault Weapons Ban, passed in 1994, further prohibited the manufacture of additional Steyr AUGs or their copies. The ban expired in 2004, and in 2008, Steyr Arms worked with Sabre Defence to produce parts legally in the U.S.

The Steyr AUG is a selective-fire, bullpup assault rifle with a conventional gas-piston-operated action that fires from a closed bolt. It is designed as a Modular Weapon System that could be quickly configured as an assault rifle, a carbine, a submachine gun and even an open-bolt light machine gun.

The AUG is chambered for the 5.56×45mm NATO cartridge and has the standard 1:9 rifling twist that will stabilise both SS109/M855 and M193 rounds. Some nations including Australia, Ireland and New Zealand use a version with a 1:7 twist optimised for the SS109 NATO round. The submachine gun variants are chambered in either 9×19mm Parabellum or .40 S&W. In Enforce Tac 2025, a Steyr AUG chambered in 7.62x39mm was unveiled.

The AUG consists of six interchangeable assemblies: the barrel, receiver with integrated telescopic sight or Picatinny rail, bolt carrier assembly, trigger mechanism, stock and magazine. The AUG employs a very high level of advanced firearms technology and is made with the extensive use of polymers and aluminium components.

The AUG comes with a muzzle cap, spare bolt for left-handed shooters, blank-firing adaptor, cleaning kit, sling and either an American M7 or German KCB-77 M1 bayonet.^{[citation needed]}