Combat shotguns, shotguns modified slightly to increase their suitability for use in combat, have been popular in the United States, both with law enforcement and with the US military (particularly the US Marine Corps), since the 19th Century.

The most common type of shotgun used for this purpose is the manually-operated, slide-action/pump-action type, because it is less prone to malfunction, particularly when dirty, than semi-automatic designs. Combat shotguns typically have much shorter barrels than shotguns for hunting and usually, though not always, have magazines of modified design to hold more than the three to five shots normal with sporting or hunting shotguns. Most combat shotguns have tubular magazines to hold the cartridges, mounted underneath the barrel, identical to those of hunting shotguns except for being longer to hold more ammunition, though some recent designs have detachable box magazines.

In law enforcement use, weapons of this type are most often called "riot guns" because, before around 1930, it was commonplace for police to attack unruly crowds of people who failed to disperse after being "read the riot act" by firing shotguns loaded with ammunition containing small "birdshot" pellets at the ground just in front of them, so that pellets would ricochet upwards and strike the rioters in the legs and groin. This tactic became less common as chemical agents like tear gas became more common in police arsenals after the First World War, but the term "riot gun" is still in very common use, particularly among police officers in the US.

Combat shotguns for military use are typically very similar to those for police use, but usually have a ventilated steel or plastic handguard over the barrel to reduce the danger that a soldier will burn his hand on the hot barrel during rapid fire, and usually also have attachment hardware to mount a bayonet under the muzzle.

The most common type of ammunition used in combat shotguns, whether for military or law enforcement purposes, is buckshot. Buckshot ammunition contains several large individual lead bullets instead of the many tiny lead or steel pellets used in birdshot. Buckshot is also used for hunting large game, up to the size of bear or deer, whereas birdshot is used for small game, such as birds, or for shooting at clay targets in games like skeet. In military use, flechette ammunition has also been used in shotguns, but not commonly due to its high cost of manufacture. Other experimental shotgun ammunition has been created, such as SCIMTR, and shotguns can fire special-purpose ammunition, such as breaching rounds.

Combat shotguns are favoured partly because of their ability to rapidly disable a person by either killing or severely wounding them.

See also: Riot shotgun