Which Istanbul landmark was completed as a Byzantine cathedral in 537?

The Istanbul landmark completed as a Byzantine cathedral in 537 was Hagia Sophia.

The present Hagia Sophia was built in Constantinople under the Byzantine emperor Justinian I after an earlier church on the site was destroyed during the Nika riots. Designed by Isidore of Miletus and Anthemius of Tralles, the basilica was formally inaugurated on 27 December 537 and became the cathedral of Constantinople.

Its vast dome, supported by pendentives, was a remarkable engineering achievement. For centuries, Hagia Sophia was the largest church in the world and the central setting for Byzantine imperial ceremonies. Earthquakes damaged the dome soon after completion, leading to rebuilding and structural changes that helped create the form seen today.

After Constantinople fell to the Ottoman Empire in 1453, the building became a mosque and received minarets. It served as a museum from 1935 until 2020, when it was redesignated as a mosque. “Hagia Sophia” means “Holy Wisdom”; it is the building’s historic name, not the name of a saint.