Istanbul. Emperor Constantine made it the capital in about 330. It was at first called New Rome and later Byzantium. For many centuries it was also called Constantinople.
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The last capital of the western part of the Roman Empire was Ravenna. It replaced Milan as the capital on 402.
The Emperor Constantine the Great (also known as Constantine I) moved the capital of the old Roman Empire to the Ancient Greek city in the East called Byzantium, and renamed this city as "Constantinople - the New Rome" in the year 330 AD. Constantinople remained the capital of the New Roman (or Eastern Roman) Empire until the year 1453 AD.
Well, there were many. The most famous one is of course Rome, which was the capital until the fourth century, then for the whole fourth century Milan was the new capital, followed by Ravenna in 402 AD, and so on.
Byzantium, renamed Constantinople.
The capital of the Roman Empire was Rome until 284. In that year the emperor Diocletian created a co-emperorship with himself in charge of the eastern part of the empire and Maximian in charge of the western part. He also designated also an imperial capital for the eastern part of the empire, which was Nicomedia (in northwestern Turkey), and an imperial capital for the western part, which was Milan (in northern Italy). The emperor Constantine I moved the capital of the eastern part to the nearby Byzantium, which he renamed Constantinople (present day Istanbul). The capital of the western part was moved to Ravenna (also in northern Italy) in 402.