Byzantine architecture is the architecture of the Byzantine Empire, or Eastern Roman Empire, usually dated from AD 330, when Constantine the Great established a new Roman capital in Byzantium, which became Constantinople, until the fall of the Byzantine Empire in 1453. There was initially no hard line between the Byzantine and Roman Empires, and early Byzantine architecture is stylistically and structurally indistinguishable from late Roman architecture. The style continued to be based on arches, vaults and domes, often on a large scale. Wall mosaics with gold background became standard for the grandest buildings, with frescos a cheaper alternative.

The richest interiors were finished with thin plates of marble or coloured and patterned stone. Some of the columns were also made of marble. Other widely used materials were bricks and stone. Mosaics made of stone or glass tesserae were also elements of interior architecture. Precious wood furniture, like beds, chairs, stools, tables, bookshelves and silver or golden cups with beautiful reliefs, decorated Byzantine interiors.

Early Byzantine architecture drew upon earlier elements of Roman and Greek architecture. Stylistic drift, technological advancement, and political and territorial changes meant that a distinct style gradually resulted in the Greek cross plan in church architecture. Civil architecture continued Greco-Roman trends; the Byzantines built impressive fortifications and bridges, but generally not aqueducts on the same scale as the Romans.

This terminology was introduced by modern historians to designate the medieval Roman Empire as it evolved as a distinct artistic and cultural entity centered on the new capital of Constantinople (modern-day Istanbul) rather than the city of Rome and its environs. Its architecture dramatically influenced the later medieval architecture throughout Europe and the Near East.

When the Roman Empire became Christian (after having extended eastwards) with its new capital at Constantinople, its architecture became more sensuous and ambitious. This new style with exotic domes and richer mosaics would come to be known as "Byzantine" before it traveled west to Ravenna and Venice and as far north as Moscow. Most of the churches and basilicas have high-riding domes, which created vast open spaces at the centers of churches, thereby heightening the light. The round arch is a fundamental of Byzantine style. Magnificent golden mosaics with their graphic simplicity brought light and warmth into the heart of churches. Byzantine capitals break away from the Classical conventions of ancient Greece and Rome with sinuous lines and naturalistic forms, which are precursors to the Gothic style.

In the same way the Parthenon is the most impressive monument for Classical religion, Hagia Sophia remained the iconic church for Christianity. The temples of these two religions differ substantially from the point of view of their interiors and exteriors. For Classical temples, only the exterior was important, because only the priests entered the interior, where the statue of the deity to whom the temple was dedicated was kept. The ceremonies were held outside, in front of the temple. Instead, Christian liturgies were held inside the churches.

Byzantine columns are quite varied, mostly developing from the classical Corinthian, with the ornamentation undercut with drills, and fluted shafts almost entirely abandoned. The block of stone was left rough as it came from the quarry, and the sculptor evolved new designs to his own fancy, so that one rarely meets with many repetitions of the same design. One of the most remarkable designs features leaves carved as if blown by the wind; the finest example being at the 7th-century Hagia Sophia (Thessaloniki). Those in the Cathedral of Saint Mark, Venice (1071) specially attracted John Ruskin's fancy. Others appear in Sant'Apollinare in Classe, Ravenna (549).

The column in San Vitale, Ravenna (547) shows above it the dosseret required to carry the arch, the springing of which was much wider than the abacus of the column. On eastern columns the eagle, the lion and the lamb are occasionally carved, but treated conventionally.