The Bosporus or Bosphorus Strait (/ˈbɒspərəs, ˈbɒsfərəs/ BOSS-pər-əs, BOSS-fər-əs; Turkish: İstanbul Boğazı, lit. 'Istanbul strait', colloquially Boğaz) is a natural strait and an internationally significant waterway located in Turkey which is straddled by the city of Istanbul. The Bosporus connects the Black Sea to the Sea of Marmara and forms one of the continental boundaries between Asia and Europe. It also divides Turkey by separating Asia Minor 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.

The name of the strait comes from the Ancient Greek Βόσπορος (Bósporos), which was folk-etymologised as βοὸς πόρος, i.e. "cattle strait" (like "Ox-ford"), from the genitive of boûs βοῦς 'ox, cattle' + poros πόρος 'passage', thus meaning 'cattle-passage', or 'cow passage'. This is a reference to the Greek mythological story of Io, who was transformed into a cow and condemned to wander the Earth until she crossed the Bosporus, where she met the Titan Prometheus, who comforted her by telling her that she would be restored to human form by Zeus and become the ancestor of the greatest of all heroes, Heracles (Hercules).

Io supposedly went ashore near Chrysopolis (present-day Üsküdar), which was named Bous 'the Cow'. The same site was also known as Damalis (Δάμαλις), as it was where the Athenian general Chares had erected a monument to his wife Damalis, which included a colossal statue of a cow (the name δαμάλις translating to 'heifer').

The English spelling with -ph- (Bosphor) is not justified by the ancient Greek name, and dictionaries prefer the spelling with -p-. However -ph- occurs as a variant in medieval Latin (as Bosphor, and occasionally Bosphorus or Bospherus), and in medieval Greek sometimes as Βόσφορος, giving rise to the French Bosphore, the Spanish Bósforo, the Italian Bosforo and the Russian Босфор. The 12th-century Greek scholar John Tzetzes calls it Damaliten Bosporon (after Damalis), but he also reports that in popular usage the strait was known as Prosphorion during his day, the name of the most ancient northern harbour of Constantinople. In English, the preferred spelling tends to be Bosphorus.

Historically, the Bosporus was also known as the "Strait of Constantinople", or the Thracian Bosporus to distinguish it from the Cimmerian Bosporus in Crimea. These are expressed in Herodotus's Histories, 4.83; as Bosporus Thracius, Bosporus Thraciae, and Βόσπορος Θρᾴκιος (Bósporos Thráikios), respectively. Other names used by Herodotus to refer to the strait include Chalcedonian Bosporus (Bosporus Chalcedoniae, Βόσπορος τῆς Χαλκηδονίης [Bosporos tes Khalkedonies], Herodotus 4.87), or Mysian Bosporus (Bosporus Mysius).