The Bosphorus is the strait separating Europe from Asia at Istanbul.
The Bosphorus is a narrow waterway linking the Black Sea in the north with the Sea of Marmara in the south. Istanbul occupies both of its shores, making it one of the world’s most prominent transcontinental cities. The strait is approximately 31 kilometers long, although its width varies considerably.
The waterway has been strategically important since antiquity because it controls maritime access between the Black Sea and the Mediterranean. Ancient Greek settlements, Byzantium, Constantinople, and modern Istanbul all benefited from its position. Two major bridges and the Eurasia Tunnel now connect the European and Asian sides of the city.
The Bosphorus is often confused with the Dardanelles. The Dardanelles lies farther southwest and connects the Sea of Marmara with the Aegean Sea. Together, the Bosphorus, Sea of Marmara, and Dardanelles form the Turkish Straits system. Under the Montreux Convention of 1936, Turkey regulates passage through the straits under specified conditions.