Hagia Sophia, officially the Hagia Sophia Grand Mosque, 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 an Eastern rite 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.