The Bosporus or Bosphorus Strait (/ˈbɒspərəs, ˈbɒsfərəs/ BOSS-pər-əs, BOSS-fər-əs;[a] Ancient Greek: Βόσπορος, romanized: Bósporos, IPA: [bósporos]; Turkish: İstanbul Boğazı, lit. 'Istanbul strait', colloquially Boğaz) is a natural strait and an internationally significant waterway located in Istanbul in northwestern Turkey that connects the Black Sea to the Sea of Marmara. It forms part of the continental boundary between Asia and Europe, and divides Turkey by separating Anatolia from Thrace. It is the world's narrowest strait used for international navigation.
Most of the shores of the Bosporus Strait, except for the area to the north, are heavily settled, with the city of Istanbul's metropolitan population of 17 million inhabitants extending inland from both banks.
The Bosporus Strait and the Dardanelles Strait at the opposite end of the Sea of Marmara are together known as the Turkish Straits.
Sections of the shore of the Bosporus in Istanbul have been reinforced with concrete or rubble and those sections of the Strait prone to deposition are periodically dredged.