A submachine gun (SMG) is an air-cooled, magazine-fed, automatic carbine designed to fire pistol cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun.

The submachine gun was developed during World War I (1914–1918). At its zenith during World War II (1939–1945), millions of SMGs were made. After the war, new SMG designs appeared almost every week. However, by the 1980s, SMG usage decreased. Today, submachine guns have been largely replaced by assault rifles, which have a greater effective range and are capable of penetrating the helmets and body armor used by modern infantrymen. However, submachine guns are still used by some military special forces, such as the United States Navy SEALs, because of their smaller size and because they are easily sound suppressed. Also, SMG's are still widely used by police SWAT teams and other militarized police, because their pistol caliber bullets are less likely to shoot through suspects and walls, causing unintentional injuries to innocent bystanders.