Chuuk Lagoon, previously Truk Atoll, is an atoll in the central Pacific. It lies about 1,800 kilometres northeast of New Guinea and is part of Chuuk State within the Federated States of Micronesia (FSM). A protective reef, 225 kilometres (140 mi) around, encloses a natural harbour 79 by 50 km, with an area of 2,130 km2 (820 sq mi). It has a land area of 93.07 square kilometres, with a population of 36,158 people and a maximal elevation of 443 metres (1,453 ft). Weno city on Weno Island functions as both the atoll's capital and the state capital, and is the largest city in the FSM with its 13,700 people.

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.