An ocean current is any more or less permanent or continuous, directed movement of ocean water that flows in one of the Earth's oceans. Ocean currents can flow for thousands of kilometers. They are very important in determining the climates of the continents, especially those regions bordering on the ocean. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude. Another example is the Hawaiian Islands, where the climate is somewhat cooler (sub-tropical) than the tropical latitudes in which they are located because of the California Current. However the importance of currents is also illustrated by the El Niño, in which a temporary reversal of an ocean current causes devastating climatic changes along the west coast of South America. El Niño effects spread as far as Australia.

Currents that flow under the surface of the ocean, and are thus hidden from immediate detection, are called submarine rivers.

Ocean currents are also very important in the dispersal of many life forms. A dramatic example is the life-cycle of the eel.

Important currents include: