The Bosphorus (/ˈbɒsfərəs/ or /ˈbɒspərəs/) or Bosporus (/ˈbɒspərəs/; Ancient Greek: Βόσφορος, Bósphoros; Turkish: Boğaziçi, pronounced [boːaziˈt͡ʃi]) is a narrow, natural strait and internationally significant waterway located in northwestern Turkey. It forms part of the continental boundary between Europe and Asia, and separates Asian Turkey from European Turkey. The world's narrowest strait used for international navigation, the Bosphorus connects the Black Sea with the Sea of Marmara, and, by extension via the Dardanelles, the Aegean and Mediterranean Seas.

Most of the shores of the strait are heavily settled, straddled by the city of Istanbul's metropolitan population of 17 million inhabitants extending inland from both coasts.

Together with the Dardanelles, the Bosphorus forms the Turkish Straits.