The Pacific Ocean contains an estimated 20,000 to 30,000 islands (the exact number has yet to be precisely determined). Those islands lying south of the tropic of Cancer but excluding Australia are traditionally grouped into three divisions: Melanesia, Micronesia, and Polynesia. Inhabitants are sometimes referred to as Pacific Islanders.

Pacific islands are also sometimes collectively called Oceania (although Oceania is sometimes defined as also including Australasia and the Malay Archipelago),

Melanesia means black islands. These include New Guinea (the largest Pacific island, which is divided into the sovereign nation of Papua New Guinea and the Indonesian provinces of Papua and West Papua), New Caledonia, Vanuatu, Fiji, and the Solomon Islands.

Micronesia means small islands. These include the Marianas, Guam, Wake Island, Palau, the Marshall Islands, Kiribati, Nauru, and the Federated States of Micronesia. Most of these lie north of the equator. Polynesia means many islands. These include New Zealand, the Hawaiian Islands, Rotuma, the Midway Islands, Samoa, American Samoa, Tonga, Tuvalu, the Cook Islands, French Polynesia, and Easter Island. It is the largest of the three zones.

The region's islands are classified into two groups, high islands and low islands. Volcanoes form high islands, which generally can support more people and have a more fertile soil. Low islands are reefs or atolls, and are relatively small and infertile. Melanesia, the most populated of the three regions, contains mainly high islands, while most of Micronesia and Polynesia are low islands.

There are also many other islands located within the boundaries of the Pacific Ocean, but these are not considered part of Oceania. These islands include the Galápagos Islands of Ecuador; the Aleutian Islands in Alaska; the Russian islands of Sakhalin and Kuril Islands; Taiwan; the Philippines; the South China Sea Islands; most of the islands of Indonesia; and the island nation of Japan, which includes the Ryukyu Islands. The inhabitants of these islands are not considered to be Pacific Islanders and are usually identified with their nearest continent.