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, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions. This ocean zone is where cold, northward flowing waters from the Antarctic mix with warmer subantarctic waters.

Geographers disagree on the Southern Ocean's northern boundary or even its existence, with many considering the waters part of the Pacific, Atlantic, and Indian Oceans instead. Others regard the Antarctic Convergence, an ocean zone which fluctuates seasonally, as separating the Southern Ocean from other oceans, rather than the 60th parallel.Australian authorities regard the Southern Ocean as lying immediately south of Australia.

The International Hydrographic Organization (IHO) has not yet ratified its 2000 definition of the ocean as being south of 60°S. Its latest published definition of oceans dates from 1953; this does not include the Southern Ocean. However, the more recent definition is used by the IHO and others.