Indian Ocean, an ocean lying south of Asia, west of Australia, north of Antarctica, and east of Africa. One-fourth of the world's people live in countries touching this ocean. The area of the Indian Ocean is 28,350,500 square miles (73,427,000 km2). It covers one-seventh of the earth's surface but is slightly smaller than the Atlantic Ocean and is less than half as large as the Pacific. Its greatest depth, 25,344 feet (7,725 m), is in Java Trench. The average depth is about 12,800 feet (3,900 m). Madagascar is the largest island.

Bass Strait, between Tasmania and southern Australia, and passages through the East Indies are important routes to the Pacific Ocean on the east. The Atlantic lies west of Africa. To the north, the Indian subcontinent divides the ocean into the Bay of Bengal on the east and the Arabian Sea on the west. The Persian Gulf and the Red Sea are arms of the Arabian Sea. The Suez Canal (at the head of the Red Sea) leads to the Mediterranean Sea.

Scylax of Caryanda, a Greek geographer of the sixth century b.c., was probably the first European to sail the Indian Ocean. In 1498 Vasco da Gama of Portugal crossed it to reach India. Its northern portion became a shipping lane between Europe and the Far East after the Suez Canal opened in 1869.

The first systematic survey of this little-studied ocean was made by the International Indian Ocean Expedition of 1959–65.