A barrel vault, also known as a tunnel vault, wagon vault or wagonhead vault, is an architectural element formed by the extrusion of a single curve (or pair of curves, in the case of a pointed barrel vault) along a given distance. The curves are typically circular in shape, lending a semi-cylindrical appearance to the total design. The barrel vault is the simplest form of a vault: effectively a series of arches placed side by side (i.e., one after another). It is a form of barrel roof.

As with all arch-based constructions, there is an outward thrust generated against the walls underneath a barrel vault. There are several mechanisms for absorbing this thrust. One is to make the walls exceedingly thick and strong – this is a primitive and sometimes unacceptable method. A more elegant method is to build two or more vaults parallel to each other; the forces of their outward thrusts will thus negate each other. This method was most often used in construction of churches, where several vaulted naves ran parallel down the length of the building. However, the outer walls of the outermost vault would still have to be quite strong or reinforced by buttressing. The third and most elegant mechanism to resist the lateral thrust was to create an intersection of two barrel vaults at right angles, thus forming a groin vault.

Barrel vaults are known from the Ancient Iran (Elam), Indian Indus Valley Civilization and Ancient Egypt, and were used extensively in Roman architecture. They were also used to replace the Cloaca Maxima with a system of underground sewers. Other early barrel vault designs occur in northern Europe, Turkey, Morocco, and other regions. In medieval Europe, the barrel vault was an important element of stone construction in monasteries, castles, tower houses and other structures. This form of design is observed in cellars, crypts, long hallways, cloisters and even great halls.

Barrel vaulting was known and utilized by early civilizations, including ancient Egypt and Mesopotamia. However, it apparently was not a very popular or common method of construction within these civilizations. The Persians and the Romans were the first to make significant architectural use of them. The technique probably evolved out of necessity to roof buildings with masonry elements such as bricks or stone blocks in areas where timber and wood were scarce. The earliest known example of a vault is a tunnel vault found under the Sumerian ziggurat at Nippur in Babylonia, ascribed to about 4000 BC, which was built from fired bricks amalgamated with clay mortar. The earliest tunnel vaults in Egypt are found at Requagnah and Denderah, from around 3500 BC in the predynastic era. These were built with sun-dried brick in three rings over passages descending to tombs with a span of only two metres. In these early instances, the barrel vault was chiefly used for underground structures such as drains and sewers, though several buildings of the great Late Egyptian mortuary palace-temple of Ramesseum were also vaulted in this way. Recent archaeological evidence discovered at the Morgantina site (in the province of Enna) shows that the aboveground barrel vault was known and used in Hellenistic Sicily in 3rd century BC, indicating that the technique was also known to ancient Greeks.

The vaulted roof of an early Harappan burial chamber has been noted from Rakhigarhi. S.R Rao reports vaulted roof of a small chamber in a house from Lothal. Barrel vaults were also used in the Late Harappan Cemetery H culture dated 1900 BC-1300 BC which formed the roof of the metal working furnace, the discovery was made by Vats in 1940 during excavation at Harappa.

Ancient Romans most probably inherited their knowledge of barrel vaulting from Etruscans and the Near East. Persians and Romans were the first to use this building method extensively on large-scale projects and were probably the first to use scaffolding to aid them in construction of vaults spanning over widths greater than anything seen before. However, Roman builders gradually began to prefer the use of groin vault; though more complex to erect, this type of vault did not require heavy, thick walls for support (see below), and thus allowed for more spacious buildings with greater openings and much more light inside, such as thermae.

After the fall of the Roman Empire, few buildings large enough to require much in the way of vaulting were built for several centuries. In the early Romanesque period, a return to stone barrel vaults was seen for the first great cathedrals; their interiors were fairly dark, due to thick, heavy walls needed to support the vault. One of the largest and most famous churches enclosed from above by a vast barrel vault was the church of Cluny Abbey, built between the 11th and 12th centuries.

In 13th and 14th centuries, with the advance of the new Gothic style, barrel vaulting became almost extinct in constructions of great Gothic cathedrals; groin vaults reinforced by stone ribs were mostly used in the beginning, and later on various types of spectacular, ornate and complex medieval vaults were developed. However, with the coming of the Renaissance and the Baroque style, and revived interest in art and architecture of antiquity, barrel vaulting was re-introduced on a truly grandiose scale, and employed in the construction of many famous buildings and churches, such as Basilica di Sant'Andrea di Mantova by Leone Battista Alberti, San Giorgio Maggiore by Andrea Palladio, and perhaps most glorious of all, St. Peter's Basilica in Rome, where a huge barrel vault spans the 27 m (89 ft)-wide nave.