Hagia Sophia (lit. 'Holy Wisdom'; Turkish: Ayasofya; Greek: Ἁγία Σοφία, romanized: Hagía Sofía; Latin: Sancta Sapientia), officially the Hagia Sophia Grand Mosque (Turkish: Ayasofya-i Kebir Cami-i Şerifi), is a mosque and former church serving as a major cultural and historical site in Istanbul, Turkey. The last of three church buildings to be successively erected on the site by the Eastern Roman Empire, it was completed in AD 537. The site was a Greek Orthodox church from AD 360 to 1453, except for a brief time as a Latin Catholic church between the Fourth Crusade and 1261. After the fall of Constantinople in 1453, it served as a mosque until 1935, when it became a museum. In 2020, the site once again became a mosque.

The current structure was built by the Byzantine emperor Justinian I as the Christian cathedral of Constantinople for the Byzantine Empire between 532 and 537, and was designed by the Greek geometers Isidore of Miletus and Anthemius of Tralles. It was formally called the Church of God's Holy Wisdom (Greek: Ναὸς τῆς Ἁγίας τοῦ Θεοῦ Σοφίας, romanized: Naòs tês Hagías toû Theoû Sophías) and upon completion became the world's largest interior space and among the first to employ a fully pendentive dome. It is considered the epitome of Byzantine architecture and is said to have "changed the history of architecture". The present Justinianic building was the third church of the same name to occupy the site, as the prior one had been destroyed in the Nika riots. As the episcopal see of the ecumenical patriarch of Constantinople, it remained the world's largest cathedral for nearly a thousand years, until the Seville Cathedral was completed in 1520.