Hagia Sophia was completed in 537 as the principal cathedral of Constantinople, capital of the Byzantine Empire. It was the third major church to occupy the site. Emperor Justinian I ordered the new building after its predecessor burned during the Nika revolt of 532. The mathematicians and architects Anthemius of Tralles and Isidore of Miletus created an immense domed interior supported by piers, arches, and pendentives. Their design made the central dome appear to float above a ring of windows and joined Roman engineering to a new vision of Christian imperial space.

The first dome partially collapsed after earthquakes in 558. Isidore the Younger rebuilt it with a steeper profile, and the church was rededicated in 562. Further earthquakes required later repairs, while emperors, patriarchs, and donors added marble revetment, liturgical furnishings, mosaics, and subsidiary spaces. Hagia Sophia served as the seat of the patriarch of Constantinople and the setting for imperial coronations, major ceremonies, and public worship. Its architecture influenced churches across the Byzantine world and projected the union of Orthodox Christianity and imperial authority.

The building also records religious and political conflict. During Byzantine Iconoclasm, figural images were removed or covered, and later generations created new mosaics of Christ, the Virgin, emperors, and empresses. Crusaders captured Constantinople in 1204 and used Hagia Sophia as a Latin Catholic cathedral until the Byzantine recovery of 1261. The occupation, looting, and later economic weakness damaged the church, but it remained the spiritual center of the shrinking empire until the Ottoman conquest.

Sultan Mehmed II converted Hagia Sophia into a mosque after taking Constantinople in 1453. Islamic worship required a mihrab, minbar, and other furnishings, while minarets and supporting structures were added outside. The architect Mimar Sinan strengthened the building with buttresses and designed additions during the sixteenth century. Many Christian mosaics were plastered or concealed rather than destroyed, and Ottoman calligraphy, lamps, tiles, and enormous medallions created another historical layer within the same interior.

The Turkish government secularized the building in 1934, and Hagia Sophia opened as a museum in 1935. Conservation exposed important Byzantine mosaics while retaining Ottoman features, allowing the structure to be interpreted as a monument to both civilizations. In 2020 it was reconverted to a mosque. Christian images in areas used for prayer are screened at certain times, while the building continues to receive visitors. Hagia Sophia’s significance lies in its exceptional architecture and in the visible succession of Byzantine cathedral, Ottoman mosque, republican museum, and active mosque within one continuously contested landmark.