The Bosporus or Bosphorus, also known as the Istanbul Strait, (Turkish: İstanbul Boğazı) (Greek: Βόσπορος from Βους [bous], the cow, and πόρος, [poros], the crossing) is a strait that forms the boundary between the European part (Rumelia) of Turkey and its Asian part (Anatolia). The world's narrowest strait used for international navigation, it 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). It is approximately 30 km long, with a maximum width of 3,700 metres at the northern entrance, and a minimum width of 700 metres between Kandilli and Aşiyan; and 750 metres between Anadoluhisarı and Rumelihisarı. The depth varies from 36 to 124 metres in midstream. The shores of the strait are heavily populated as the city of Istanbul (with a metropolitan area in excess of 11 million inhabitants) straddles it.

Two bridges cross the Bosporus. The first, the Bosphorus Bridge, is 1074 metres long and was completed in 1973. The second, Fatih Sultan Mehmet (Bosphorus II) Bridge, is 1090 metres long, and was completed in 1988 about five kilometres north of the first bridge. Plans for a third road bridge, which will allow transit traffic to by-pass the city traffic, have been approved by the Ministry of Transportation. The bridge will be part of the so called "Northern Marmara Motorway", which will be further integrated with the existing Black Sea Coastal Highway. The location will be somewhere north of the existing two bridges, but the exact path is kept secret to avoid an early boom in land prices.

Another crossing, Marmaray, is a 13.7 kilometre-long undersea railway tunnel currently under construction and is expected to be completed in 2012. Approximately 1,400 metres of the tunnel will run under the strait, at a depth of about 55 metres.