An atoll (/ˈæt.ɒl, -ɔːl, -oʊl, əˈtɒl, -ˈtɔːl, -ˈtoʊl/) is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most of the approximately 440 atolls in the world are in the Pacific Ocean.

Two different, well-cited models, the subsidence model and the antecedent karst model, have been used to explain the development of atolls. According to Charles Darwin's subsidence model, the formation of an atoll is explained by the sinking of a volcanic island around which a coral fringing reef has formed. Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. Eventually, reef and the small coral islets on top of it are all that is left of the original island, and a lagoon has taken the place of the former volcano. The lagoon is not the former volcanic crater. For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (sinking of the island or rising oceans).

An alternative model for the origin of atolls is called the antecedent karst model. In the antecedent karst model, the first step in the formation of an atoll is the development of a flat top, mound-like coral reef during the subsidence of an oceanic island of either volcanic or nonvolcanic origin below sea level. Then, when relative sea level drops below the level of the flat surface of coral reef, it is exposed to the atmosphere as a flat topped island which is dissolved by rainfall to form limestone karst. Because of hydrologic properties of this karst, the rate of dissolution of the exposed coral is lowest along its rim and the rate of dissolution increases inward to its maximum at the center of the island. As a result, a saucer shaped island with a raised rim forms. When relative sea level submerges the island again, the rim provides a rocky core on which coral grow again to form the islands of an atoll and the flooded bottom of the saucer forms the lagoon within them.

The word atoll comes from the Dhivehi word atholhu (އަތޮޅު, pronounced [ˈat̪oɭu]). Dhivehi is an Indo-Aryan language spoken in the Maldives. The word's first recorded English use was in 1625 as atollon. Charles Darwin coined the term in his monograph, The Structure and Distribution of Coral Reefs. He recognized the word's indigenous origin and defined it as a "circular group of coral islets", synonymously with "lagoon-island".

More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus" or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".

There are approximately 440 atolls in the world. Most of the world's atolls are in the Pacific Ocean (with concentrations in the Caroline Islands, the Coral Sea Islands, the Marshall Islands, the Tuamotu Islands, Kiribati, Tokelau, and Tuvalu) and the Indian Ocean (the Chagos Archipelago, Lakshadweep, the atolls of the Maldives, and the Outer Islands of Seychelles). In addition, Indonesia also has several atolls spread across the archipelago, such as in the Thousand Islands, Taka Bonerate Islands, and atolls in the Raja Ampat Islands. The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean.

Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The northernmost atoll in the world is Kure Atoll at 28°25′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls in the world are Elizabeth Reef at 29°57′ S, and nearby Middleton Reef at 29°27′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory. The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°41′ S. The atoll closest to the Equator is Aranuka of Kiribati. Its southern tip is just 13 km (8 mi) north of the Equator.

Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°18′ N. At this latitude, coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different origin of formation.