A casemate is a fortified gun emplacement or armoured structure from which guns are fired, in a fortification, warship, or armoured fighting vehicle.

When referring to antiquity, the term "casemate wall" means a double city wall with the space between the walls separated into chambers, which could be filled up to better withstand battering rams in case of siege (see § Antiquity: casemate wall.)

In its original early modern meaning, the term referred to a vaulted chamber in a fort, which may have been used for storage, accommodation, or artillery which could fire through an opening or embrasure. Although the outward faces of brick or masonry casemates proved vulnerable to advances in artillery performance, the invention of reinforced concrete allowed newer designs to be produced well into the 20th century. With the introduction of ironclad warships, the definition was widened to include a protected space for guns in a ship, either within the hull or in the lower part of the superstructure. Although the main armament of ships quickly began to be mounted in revolving gun turrets, secondary batteries continued to be mounted in casemates; however, several disadvantages eventually also led to their replacement by turrets. In tanks and other armored fighting vehicles lacking a turret for the main gun, the structure accommodating the gun is also called a casemate.

First recorded in French in the mid-16th century, from the Italian casamatta or Spanish casamata, perhaps meaning a slaughterhouse, although it could derive from casa (in the sense of "hut"), and matta (Latin matta), "done with reeds and wickers", thus a low-roof hut without windows or other openings set in marshy place. It could also come from casa matta with matta in the sense of "false". However, it may have been ultimately derived from the Greek chásmata (χάσματα), a gap or aperture.

The term casemate wall is used in the archaeology of Israel and the wider Near East, having the meaning of a double wall protecting a city or fortress, with transverse walls separating the space between the walls into chambers. These could be used as such, for storage or residential purposes, or could be filled with soil and rocks during siege in order to raise the resistance of the outer wall against battering rams. Originally thought to have been introduced to the region by the Hittites, this has been disproved by the discovery of examples predating their arrival, the earliest being at Ti'inik (Taanach) where such a wall has been dated to the 16th century BC. Casemate walls became a common type of fortification in the Southern Levant between the Middle Bronze Age (MB) and Iron Age II, being more numerous during the Iron Age and peaking in Iron Age II (10th–6th century BC). However, the construction of casemate walls had begun to be replaced by sturdier solid walls by the 9th century BC, probably due the development of more effective battering rams by the Neo-Assyrian Empire. Casemate walls could surround an entire settlement, but most only protected part of it. The three different types included freestanding casemate walls, then integrated ones where the inner wall was part of the outer buildings of the settlement, and finally filled casemate walls, where the rooms between the walls were filled with soil right away, allowing for a quick, but nevertheless stable construction of particularly high walls.

In fortifications designed to resist artillery, a casemate was originally a vaulted chamber usually constructed underneath the rampart. It was intended to be impenetrable and could be used for sheltering troops or stores. With the addition of an embrasure through the scarp face of the rampart, it could be used as a protected gun position. In bastion forts, artillery casemates were sometimes built into the flanks of bastions, but in action they quickly filled with smoke making them inoperable and for that reason, had fallen out of favor during the 17th century.

In the late 18th century, Marc René, marquis de Montalembert (1714–1800) experimented with improved casemates for artillery, with ventilation systems that overcame the problem of smoke dispersal found in earlier works. For coastal fortifications, he advocated multi-tiered batteries of guns in masonry casemates, that could bring concentrated fire to bear on passing warships. In 1778, he was commissioned to build a fort on the Île-d'Aix, defending the port of Rochefort, Charente-Maritime. The outbreak of the Anglo-French War forced him to hastily to build his casemated fort from wood but he was able to prove that his well-designed casemates were capable of operating without choking the gunners with smoke. The defenses of the new naval base at Cherbourg were later constructed according to his system. After seeing Montalembert's coastal forts, American engineer Jonathan Williams acquired a translation of his book and took it to the United States, where it inspired the Second and Third Systems of coastal fortification; the first fully developed example being Castle Williams in New York Harbor which was started in 1807.

In the early 19th century, French military engineer Baron Haxo designed a free-standing casemate that could be built on the top of the rampart, to protect guns and gunners from the high-angle fire of mortars and howitzers.