Ducie Island is an uninhabited atoll in the Pitcairn Islands group, which also includes Pitcairn, Henderson and Oeno islands. Ducie lies east of Pitcairn Island, and east of Henderson Island, and has a total area of 1.5 square miles (3.9 km2), which includes the lagoon. It is 1.5 miles (2.4 km) long, measured northeast to southwest, and about 1 mile (1.6 km) wide. The island is composed of four islets: Acadia, Pandora, Westward and Edwards.

An atoll 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.