An autocannon, automatic cannon, machine cannon or anti-tank machine gun, is a fully automatic gun that is capable of rapid-firing large-caliber (20 mm/0.79 in or more) armour-piercing, explosive or incendiary shells, as opposed to the smaller-caliber kinetic projectiles (bullets) fired by a machine gun. Autocannons have a longer effective range and greater terminal performance than machine guns, due to the use of larger/heavier munitions (most often in the range of 20–60 mm (0.79–2.36 in), but bigger calibers also exist), but are usually smaller than tank guns, howitzers, field guns, or other artillery. When used on its own, the word "autocannon" typically indicates a non-rotary weapon with a single barrel. When multiple rotating barrels are involved, such a weapon is referred to as a "rotary autocannon" or "rotary cannon". If it uses a single barrel with a rotating cylinder with multiple chambers, it is known as a "revolver autocannon" or "revolver cannon"; both of these systems are commonly used as aircraft guns and anti-aircraft guns.

Autocannons are heavy weapons that are unsuitable for use by infantry. Due to the heavy weight and recoil, they are typically installed on fixed mounts, wheeled carriages, ground combat vehicles, aircraft, or watercraft, and are almost always crew-served, or even remote-operated with automatic target recognition/acquisition (e.g. sentry guns and naval CIWS). As such, ammunition is typically fed from a belt system to reduce reloading pauses or for a faster rate of fire, but magazines remain an option. Common types of ammunition, among a wide variety, include HEIAP, HEDP and more specialised armour-piercing (AP) munitions, mainly composite rigid (APCR) and discarding sabot (APDS) rounds.

Capable of generating extremely rapid firepower, autocannons overheat quickly if used for sustained fire, and are limited by the amount of ammunition that can be carried by the weapons systems mounting them. Both the US 25 mm M242 Bushmaster and the British 30 mm RARDEN have relatively slow rates of fire so as not to deplete ammunition too quickly.^{[citation needed]} The Oerlikon KBA 25 mm has a relatively mid-high rate of fire 650 rounds per minute but can be electronically programmed to 175–200 rounds per minute. The rate of fire of a modern autocannon ranges from 90 rounds per minute, in the case of the British RARDEN, to 2,500 rounds per minute with the GIAT 30. Rotary systems with multiple barrels can achieve over 10,000 rounds per minute (the Russian GSh-6-23, for example). Such extremely high rates of fire are effectively employed by aircraft in aerial dogfights and close air support on ground targets via strafing attacks, where the target dwell time is short and weapons are typically operated in brief bursts.

The first modern autocannon was the British QF 1-pounder, also known as the "pom-pom". This was essentially an enlarged version of the Maxim gun, which was the first successful fully automatic machine gun, requiring no outside stimulus in its firing cycle other than holding the trigger. The pom-pom fired 1 pound (0.45 kg) gunpowder-filled explosive shells at a rate of over 200 rounds a minute: much faster than conventional artillery while possessing a much longer range and more firepower than the infantry rifle.

In 1913, Reinhold Becker and his Stahlwerke Becker firm designed the 20mm Becker cannon, addressing the German Empire's perceived need for heavy-calibre aircraft armament. The Imperial Government's Spandau Arsenal assisted them in perfecting the ordnance. Although only about 500+ examples of the original Becker design were made during World War I, the design's patent was acquired by the Swiss Oerlikon Contraves firm in 1924, with the Third Reich's Ikaria-Werke firm of Berlin using Oerlikon design patents in creating the MG FF wingmount cannon ordnance. The Imperial Japanese Navy's Type 99 cannon, adopted and produced in 1939, was also based on the Becker/Oerlikon design's principles.

During the First World War, autocannons were mostly used in the trenches as anti-aircraft guns. The British used pom-pom guns as part of their air defences to counter the German Zeppelin airships that made regular bombing raids on London. However, they were of little value, as their shells neither ignited the hydrogen of the Zeppelins nor caused sufficient loss of gas (and hence lift) to bring them down. Attempts to use the guns in aircraft failed, as the weight severely limited both speed and altitude, thus making successful interception impossible. The more effective QF 2 pounder naval gun would be developed during the war to serve as an anti-aircraft and close range defensive weapon for naval vessels.

Autocannons would serve to a much greater extent and effect during the Second World War. The German Panzer II light tank, which was one of the most numerous in German service during the invasion of Poland and the campaign in France, used a 20 mm autocannon as its main armament. Although ineffective against tank armour even during the early years of the war, the cannon was effective against light-skinned vehicles as well as infantry and was also used by armoured cars. Larger examples, such as the 40 mm Vickers S, were mounted in ground attack aircraft to serve as an anti-tank weapon, a role to which they were suited as tank armour is often lightest on top.

The Polish 20 mm 38 Fk auto cannon was expensive to produce, but an exception. Unlike the Oerlikon, it was effective against all the tanks fielded in 1939, largely because it was built as an upgrade to the Oerlikon, Hispano-Suiza, and Madsen. It even proved capable of knocking out early Panzer IIIs and IVs, albeit with great difficulty. Only 55 were produced by the time of the Polish Defensive War. However it was in the air war that these weapons played their most important part in the conflict.