What was the capital of the Byzantine Empire?

The capital of the Byzantine Empire was Constantinople.

Emperor Constantine the Great refounded the ancient Greek city of Byzantium as his new imperial capital in AD 324. It was formally dedicated in 330 and became known as Constantinople, or “City of Constantine.” Its position beside the Bosporus gave the city control over important routes linking the Black Sea, the Aegean, Europe, and Asia.

After the Roman Empire was permanently divided in 395, Constantinople served as the capital of the Eastern Roman Empire, which modern historians call the Byzantine Empire. It remained the political, religious, and cultural center of the empire for more than a thousand years, apart from the period of Latin occupation after the Fourth Crusade captured the city in 1204.

A common mix-up is to answer Rome, because the Byzantines considered themselves Romans and their empire a continuation of Rome. Constantinople, however, was the Byzantine capital. The city was captured by the Ottomans in 1453 and is now Istanbul.