Bosphorus also known as the Istanbul Strait (Turkish: İstanbul Boğazı), is a strait that forms part of the boundary between Europe and Asia. The Bosphorus, the Sea of Marmara, and the Dardanelles strait to the southwest together form the Turkish Straits. The world's narrowest strait used for international navigation, the Bosphorus connects the Black Sea with the Sea of Marmara (which is connected by the Dardanelles to the Aegean Sea, and thereby to the Mediterranean Sea.)
The Bosphorus' limits are defined as the connecting line between the lighthouses Rumeli Feneri and Anadolu Feneri in the north and between the Ahırkapı Feneri and the Kadıköy İnciburnu Feneri in the south. Between the limits, the strait is 31 km (17 nmi) long, with a width of 3,329 m (1.798 nmi) at the northern entrance and 2,826 m (1.526 nmi) at the southern entrance. Its maximum width is 3,420 m (1.85 nmi) between Umuryeri and Büyükdere Limanı, and minimum width 700 m (0.38 nmi) between Kandilli Point and Aşiyan. This part of the strait is a dangerous point for maritime traffic: a 45-degree course alteration is required, and the current can reach 7–8 knots. To the south, at Yenikoy, the necessary course alteration is 80 degrees. All the dangers and obstacles characteristic of narrow waterways are present and acute in this critical sea lane. At the above mentioned turns (Kandilli and Yenikoy) where significant course alterations have to be made, the rear and forward sights are totally blocked prior to and during the course alteration. Ships approaching from the opposite direction cannot be seen round these bends. The risks posed by geography are increased dramatically by the heavy ferry traffic across the strait, linking the European and Asian sides of the city.

The depth of Bosphorus varies from 36 to 124 m (118 to 407 ft) in midstream with an average of 65 m (213 ft). The deepest location is between Kandilli and Bebek with 110 m (360 ft). The most shallow locations are off Kadıköy İnciburnu on the northward route with 18 m (59 ft) and off Aşiyan Point on the southward route with 13 m (43 ft).

The shores of the strait are heavily populated, straddled as it is by the city of Istanbul (with a metropolitan area in excess of 12 million inhabitants) which extends inland from both coasts.

The name comes from Greek Bosporos which the ancient Greeks analysed as bous βοῦς 'ox' + poros πόρος 'means of passing a river, ford, ferry', thus meaning 'ox-ford', which is a reference to Io (mythology) from Greek mythology who was transformed into a cow and condemned to wander the earth until she crossed the Bosphorus where she met Prometheus. Although it has been known for a while that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow, findings of a study by the University of Leeds in August 2010 reveal that there is in fact an underwater channel of high-density water flowing across the floor of the Bosphorus (caused by the difference in density of the two seas), which would be the sixth largest river on Earth if it were to be on land.