Which two bodies of water does the Bosporus connect?

The Bosporus connects the Black Sea and the Sea of Marmara.

The Bosporus is a narrow strait running through Istanbul, Turkey. It links the Black Sea to the Sea of Marmara, which in turn connects through the Dardanelles to the Aegean Sea and the Mediterranean. This makes the strait part of the maritime route between the Black Sea and the wider global ocean system.

The waterway is about 31 kilometres long and varies considerably in width. Istanbul occupies both banks, giving the city territory in Europe and Asia. The Bosporus is therefore commonly described as one of the world’s great intercontinental waterways.

It is not the same as the Dardanelles. The Bosporus lies farther north and borders Istanbul, while the Dardanelles lies southwest of the Sea of Marmara. Together, the two straits have been strategically important since antiquity and remain governed under the 1936 Montreux Convention.