In which present-day city does the Hagia Sophia stand?

The Hagia Sophia stands in Istanbul, Türkiye. The monument occupies a historic position in the city’s Sultanahmet district, near the Blue Mosque and Topkapı Palace.

The current building was constructed under the Byzantine emperor Justinian I and was dedicated in 537. For nearly a millennium it served as the cathedral of Constantinople, the capital of the Byzantine Empire. After the Ottoman conquest of Constantinople in 1453, it became an imperial mosque and acquired minarets and other Islamic architectural features.

Hagia Sophia became a museum in 1935 and was redesignated as a mosque in 2020. Its huge dome, pendentives, mosaics and later additions show how the building changed across empires and religious traditions. The city was called Constantinople for much of its medieval and early modern history; “Istanbul” is the modern name used in the question.