Lihou Reef National Nature Reserve, Marine reserve in the Coral Sea Islands Territory, Australia

Lihou Reef is a U-shaped atoll in the Coral Sea, opening to the south-west, with a lagoon about 100 by 30 km and up to 60 m deep. Its rim holds 18 islets and cays, nine on the Northern Rim and nine on the Southern Rim, totalling barely more than one square kilometre of land. Only Lorna Cay, Turtle Islet, Anne Cay, Georgina Cay and Nellie Cay carry vegetation.

The reef was discovered in 1823 by Lieutenant John Lihou while navigating through uncharted waters on HMS Zenobia. The area later received formal protection status to conserve its marine environment.

The atoll's name honours Lieutenant John Lihou of HMS Zenobia, who sighted it on 23 February 1823; it is not the same as Lihou island in the Channel Islands. Since 16 August 2002 it has formed, with Coringa-Herald, the Coral Sea Reserves Ramsar Site, a wetland of international importance.

The reserve lies about 700 km east of Cairns, Queensland, and is reached only by boat over open ocean, so proper maritime experience is required. Entry into the lagoon is possible through Herald Passage, just northeast of Anne Cay. Turtle Islet, 6 m high, carries an automated weather station.

The lagoon spans 100 by 30 kilometers with depths reaching 60 meters, making it the second largest atoll formation in the Coral Sea.