Hagia Sophia Greek: Ἁγία Σοφία; Holy Wisdom, Turkish Ayasofya) is a former patriarchal basilica and mosque, now a museum, in Istanbul, Turkey. Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture and one of the most beautiful buildings in the world.

The building was originally constructed as a Church between 532 and 537 on the orders of the Byzantine Emperor Justinian, and was in fact the third Church of the Holy Wisdom to occupy the site (the previous two had both been destroyed by riots). It was designed by two architects, Isidore of Miletus and Anthemius of Tralles. The Church contained a large collection of holy relics and featured, among other things, a 50-foot silver iconostasis. It was the patriarchal church of the Patriarch of Constantinople and the religious focus point of the Orthodox Byzantine Empire for nearly 1000 years.

In 1453, Constantinople was conquered by the Ottoman Turks and Sultan Mehmed II ordered the building to be converted into a mosque. The bells, altar, iconostasis, and sacrificial vessels were removed, and many of the mosaics were eventually plastered over. The Islamic features - such as the four minarets outside, the mihrab and minbar - were added over the course of the Ottoman Empire It remained as a mosque until 1935, when it was converted into a museum by the secular Republic of Turkey.

For almost 500 years the principal mosque of Istanbul, Hagia Sophia served as a model for many of the Ottoman mosques such as the Şehzade Mosque, the Süleymaniye Mosque, and the Rüstem Pasha Mosque.

Although it is sometimes referred to as Saint Sophia, the Greek name in full is Church of the Holy Wisdom of God Ναός τῆς Ἁγίας τοῦ Θεοῦ Σοφίας - and it was dedicated to the Holy Wisdom of God rather than a specific saint named Sophia.