Hagia Sophia Greek: Ἁγία Σοφία; Holy Wisdom, Turkish Ayasofya) is a former patriarchal basilica, later a 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. It was the largest cathedral in the world for nearly a thousand years, until the completion of the Medieval Seville Cathedral in 1520.

The current 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 (15 m) 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.