A coral island is a type of island formed from coral detritus and associated organic material, typically as part of a coral reef which has grown to cover a far larger area under the sea. Coral islands are located in tropical and sub-tropical areas. They are built of organic material derived from the skeletons of corals and numerous other animals and plants associated with corals. Coral islands consist of low land, perhaps only a few meters above sea level, generally with coconut palms and surrounded by white coral sand beaches. They may extend dozens of kilometers and include almost any tropical limestone island whose structure is integrally part of a living or relatively recent coral reef.

Coral islands begin as volcanic islands over a hot spot. As the volcano emerges from the sea, a fringing reef grows on the outskirt of the volcano. The volcano eventually moves off the hot spot by means of plate tectonics. Once this occurs, the volcano can no longer keep up with the wave erosion and undergoes subsidence. Over geological time, a reef may reach the surface and can become a coral island, where it begins a whole new ecosystem for land-based creatures.

Most of the worlds coral islands are in the Pacific Ocean, but they are also found in the Atlantic and Indian Oceans. Examples of coral islands include The American territories of Jarvis, Baker, and Howland Islands, the Lakshadweep Islands union territory of India, and some of the islands belonging to Kiribati. Coral is important for biodiversity and the growth of fish populations, so maintaining coral reefs is important.