The Southern Ocean, also known as the Great Southern Ocean, the Antarctic Ocean and the South Polar Ocean, comprises the southernmost waters of the World Ocean south of 60° S latitude. The International Hydrographic Organization has designated the Southern Ocean as an oceanic division encircling Antarctica. Geographers disagree on the Southern Ocean's northern boundary or even its existence (see below), sometimes considering the waters part of the South Pacific, South Atlantic, and Indian Oceans instead. Some scientists consider the Antarctic Convergence, an ocean zone which fluctuates seasonally, as separating the Southern Ocean from other oceans, rather than 60° S. This ocean zone forms from the convergence of two circumpolar currents, one easterly flowing and one westerly flowing.

The International Hydrographic Organization (IHO) regards the Southern Ocean as the fourth-largest of the five principal oceanic divisions and the latest-defined one. The IHO promulgated the decision on its existence in 2000, though many mariners have long regarded the term as traditional. The Southern Ocean appeared in the IHO's Limits of Oceans and Seas second edition (1937), disappeared from the third edition (1957), and re-surfaced in the fourth edition (not yet formally adopted due to a number of unresolved disputes). This change reflects the importance placed by oceanographers on ocean currents.