The Black Sea (known as the Euxine Sea in antiquity) is an inland sea between southeastern Europe and Asia Minor. It is connected to the Mediterranean Sea by the Bosporus and the Sea of Marmara, and to the Sea of Azov by the Strait of Kerch.

There is a net inflow of seawater through the Bosporus, 200 km³ per year. There is an inflow of freshwater from the surrounding areas, especially central and middle-eastern Europe, totalling 320 km³ per year. The most important river entering the Black Sea is the Danube. The Black Sea has an area of 422,000 km² and a maximum depth of 2210 m.

Countries bordering on the Black Sea are Turkey, Bulgaria, Romania, Ukraine, Russia, Georgia. The Crimea is an Autonomous Republic of Ukraine.

Important cities along the coast include: Istanbul (formerly Constantinople and Byzantium), Burgas, Varna, Constant,a, Yalta, Odessa, Sevastopol, Kerch, Novorossiysk, Sochi, Sukhumi, Batumi, Trabzon, Samsun.

The Black Sea is the largest anoxic, or oxygen-free, marine system. This is a result of the great depth of the sea and the relatively high salinity (and therefore density) of the water at depth; freshwater and seawater mixing is limited to the uppermost 100 to 150 m, with the water below this interface (called the pycnocline) being exchanged only once every thousand years. There is therefore no significant gas exchange with the surface, and as a result decaying organic matter in the sediment consumes any available oxygen. In these anoxic conditions some extremophile microorganisms are able to use sulfate (SO42-) for oxidation of organic material, producing hydrogen sulfide (H2S) and carbon dioxide.

This mix is extremely toxic (a lungful would be fatal to a human), resulting in a sea that has almost all of its ecology living in that top layer down to a depth of approximately 180 m (600 ft). The relative lack of micro-organisms and oxygen has allowed deep-sea expeditions to recover ancient (on the order of thousands of years) human artifacts, such as boat hulls and the remains of settlements.

The steppes to the north of the Black Sea suggested as the original homeland (Urheimat) of the speakers of the Proto-Indo-European language, (PIE) the progenitor of the Indo-European language family, by some scholars (see Kurgan; others move the heartland further east towards the Caspian Sea, yet others to Anatolia).

The name 'Black Sea' (initially Pontos Axeinos, "inhospitable sea", later renamed Pontos Euxeinos, "hospitable sea" to gain the sea's good favor) was coined by the Ancient Greek navigators, because of the unusual dark color, compared with the Mediterranean Sea.

Visibility in the Black Sea is on average approximately 5 metres (15 feet), as compared to up to 35 metres (100 feet) in the Mediterranean. The land at the eastern end of the Black Sea, Colchis (now Georgia), marked for the Greeks an edge of the known world.