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The Black Sea is an inland sea bounded by southeastern Europe, the Caucasus and the Anatolian peninsula (Turkey) and is ultimately connected to the Atlantic Ocean via the Mediterranean and Aegean Seas and various straits. The Bosporus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects it to the Aegean Sea region of the Mediterranean. These waters separate eastern Europe and western Asia. The Black Sea also connects to the Sea of Azov by the Strait of Kerch.

The Black Sea has a positive water balance, which results in a net outflow of water 300 km³ per year through the Bosphorus into the Aegean Sea (part of the Mediterranean Sea). Mediterranean water flows into the Black Sea as part of a 2-way hydrological exchange. The Black Sea outflow is less salinated and cool, therefore floats over the warm, relatively more salinated Mediterranean inflow. The Black Sea also receives river water from large Eurasian fluvial systems to the north of the Sea, of which the Don, Dnieper and Danube are the most significant.

In the past, the water level has varied significantly. Depending on the water level in the basin, more or less of the surrounding shelf and associated aprons are aerially exposed. At certain critical depths, it is possible for connections with surrounding water bodies to become established. It is through the most active of these connective routes, the Turkish Straits System (TSS), that the Black Sea joins the global ocean system. When this hydrological link is not present, the Black Sea is a lake, operating independently of the global ocean system. Currently the Black Sea water level is relatively high, thus water is being exchanged with the Mediterranean. The TSS connects the Black and Aegean Seas and comprises the Strait of Bosphorus Strait (Strait of Istanbul), the Marmara sea and the Strait of Dardanelles (Strait of Canakkale). The Black Sea also connects to the Sea of Azov via the Strait of Kerch