The Rheinmetall Rh-120 is a 120 mm smoothbore tank gun designed and produced in former West Germany by the Rheinmetall Waffe Munition GmbH company. It was developed in response to Soviet advances in armour technology and development of new armoured threats. Production began in 1974, with the first version of the gun, known as the L/44 as it was 44 calibres long, used on the German Leopard 2 tank and soon produced under license for the American M1A1 Abrams and other tanks. The 120-millimetre (4.7 in) L/44 gun has a length of 5.28 metres (17.3 ft), and the gun system weighs approximately 3,317 kilograms (7,313 lb).

By 1990, the L/44 was not considered powerful enough to defeat future Soviet armour, which stimulated an effort by Rheinmetall to develop a better main armament. This first involved a 140-millimeter (5.5 in) tank gun named Neue Panzerkanone 140 ('new tank gun 140'), but later turned into a compromise which led to the development of an advanced 120 mm gun, the L/55, based on the same internal geometry as the L/44 and installed in the same breech and mount. The L/55 is 1.32 metres (4.3 ft) longer, generating increased muzzle velocity for rounds fired through it. As the L/55 retains the same barrel geometry, it can fire the same ammunition as the L/44.

The L/55 gun was retrofitted into German and Dutch Leopard 2s, and chosen as the main gun of the Spanish Leopard 2E and the Greek Leopard 2HEL. It was tested on the British Challenger 2 as a potential replacement for its rifled L30 120 mm cannon.

A variety of ammunition has been developed for use by tanks with guns based on Rheinmetall's original L/44 design. This includes a series of kinetic energy penetrators, such as the American M829 series, and high-explosive anti-tank warheads. Recent ammunition includes a range of anti-personnel rounds and demolition munitions. The LAHAT, developed in Israel, is a gun-launched anti-tank guided missile which has received interest from Germany and other Leopard 2 users. It is designed to defeat both land armour and combat helicopters. The Israelis also introduced a new anti-personnel munition which limits collateral damage by controlling the fragmentation of the projectile.