A submachine gun (SMG) or sub-gun is a magazine-fed automatic carbine designed to fire handgun cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun, to describe its design concept as an automatic firearm with notably less firepower than a machine gun (hence the prefix "sub-"). As a machine gun must fire rifle cartridges to be classified as such, submachine guns are not considered machine guns.

In the 20th century, the submachine gun was developed during World War I (1914–1918) as a close quarter offensive weapon, mainly for trench raiding. At its peak during World War II (1939–1945), millions of submachine guns were made for assault troops and auxiliaries whose doctrines emphasized close-quarter suppressive fire. New submachine gun designs appeared frequently during the Cold War, especially among special forces, commandos and mechanized infantrymen. Submachine gun usage for frontline combat decreased in the 1980s and 1990s.

There are some inconsistencies in the classification of submachine guns. British Commonwealth sources often refer to SMGs as "machine carbines". Other sources refer to SMGs as "machine pistols" because they fire pistol-caliber ammunition, for example, the MP-40 and MP5, where "MP" stands for Maschinenpistole ("submachine gun" in German, but cognate with the English term "machine pistol"). However, the term "machine pistol" is also used to describe a handgun-style firearm capable of fully automatic or burst fire, such as the Stechkin, Beretta 93R, Glock 18, and the H&K VP70. Furthermore, personal defense weapons such as the FN P90 and H&K MP7 are a subset of submachine guns.

In 1895, Hiram Maxim produced the 'miniature Maxim' which was a pistol-calibre Maxim machinegun weighing 27 lb (12.2 kg) that was sold in small quantities to various countries and tested by the US military but not adopted. In 1896, a select-fire pistol was patented by the British inventor Hugh Gabbett-Fairfax.

In April 1914, Abiel Bethel Revelli, an Italian military officer, patented a twin-barreled, magazine-fed automatic gun in a pistol caliber, lighter than a machine gun and shorter than a rifle. A common myth is that this weapon was originally designed as an aircraft gun. In reality ground use was taken into consideration from the very beginning, particularly for the Bersaglieri's cyclist battalions.

Stocked pistols were common at the beginning of the 20th century, the Germans initially using heavier versions of the P08 pistol equipped with a detachable stock, larger-capacity snail-drum magazine and a longer barrel.

In 1915, the Kingdom of Italy adopted Revelli's design as the FIAT Mod. 1915. It fired pistol-caliber 9mm Glisenti ammunition, but was not a true submachine gun, as it was originally designed as a mounted weapon.

In late 1915, the first submachine gun with a buttstock was built: the Austro-Hungarian Standschütze Hellriegel M1915 although the weapon was never used in combat.