The World Ocean, world ocean, or global ocean is the interconnected system of the Earth's oceanic (or marine) waters, and comprises the bulk of the hydrosphere.

The unity and continuity of the World Ocean, with relatively free interchange among its parts, is of fundamental importance to oceanography. Customarily, it is divided into a number of principal oceanic areas that are delimited by the continents and various oceanographic features: these divisions are the Atlantic Ocean, Arctic Ocean (sometimes considered a sea of the Atlantic), Indian Ocean, Pacific Ocean, and Southern Ocean (sometimes reckoned instead as just the southern portions of the Atlantic, Indian, and Pacific Oceans). In turn, oceanic waters are interspersed by many smaller seas and other bodies of water.

A global ocean has existed in one form or another on Earth for eons, and the notion dates back to classical antiquity (in the form of Oceanus). The contemporary concept of the World Ocean was coined by the Russian oceanographer Yuly Shokalsky in the early 20th century to describe what is basically a solitary, continuous ocean that covers and encircles most of the Earth.

While continuous, the World Ocean can be visualized as being centered on the Southern Ocean. The Atlantic, Indian, and Pacific Oceans can be seen as bays or lobes extending northward from the Southern Ocean. Further north, the Atlantic opens into the Arctic Ocean, which is connected to the Pacific by the Bering Strait:

• The Southern Ocean is the ocean surrounding Antarctica, dominated by the Antarctic Circumpolar Current, generally the ocean south of sixty degrees south latitude. The Southern Ocean is partially covered in sea ice, the extent of which varies according to the season. The Southern Ocean is the second smallest of the five named oceans.
• The Atlantic Ocean, the second largest, extends from the Southern Ocean between South America, Africa, North America and Europe, to the Arctic Ocean. The Atlantic meets the Indian Ocean south of Africa at Cape Agulhas.
• The Indian Ocean extends northward from the Southern Ocean to India, between Africa and Australia. The Indian Ocean joins the Pacific Ocean to the west, near Australia.
• The Pacific Ocean, the largest of all, also reaches northward from the Southern Ocean to the Arctic Ocean. It spans the gap between Australia, Asia, North America and Oceania. The Pacific Ocean meets the Atlantic south of South America at Cape Horn.
• The Arctic Ocean is the smallest of the five. It joins the Atlantic near Greenland and Iceland, and joins the Pacific at the Bering Strait. It overlies the North Pole, touching North America in the Western hemisphere and Scandinavia and Asia in the Eastern hemisphere. The Arctic Ocean is partially covered in sea ice, the extent of which varies according to the season. Some authorities do not consider the Arctic Ocean a bona fide ocean, because it is largely surrounded by land with only limited exchange of water with the other oceans. Consequently, it is considered by some to be a sea of the Atlantic, referred to as the Arctic Mediterranean Sea or Arctic Sea.

The approximate shape of the World Ocean can for most purposes be treated as constant, although in fact it is not: continental drift continually changes its structure.