Cartier Island is an uninhabited and unvegetated sand cay in a platform reef in the Timor Sea north of Australia and south of Indonesia. It is located at 12°31'S 123°33'E, on the edge of the Sahul Shelf, about 300 kilometres off the north west coast of Western Australia, 200 kilometres south of the Indonesian island of Roti, and 70 kilometres south-east of Ashmore Reef. It is within the Territory of Ashmore and Cartier Islands, an external territory of Australia, whose authority over the islands is disputed by Indonesia.
The area within of the centre of the reef is protected as the Cartier Island Marine Reserve. At the southern edge of the reef is a shipwreck of the Ann Millicent, an iron-hulled barge of 944 tons wrecked in 1888. The remains of an RAAF Beaufighter can also be seen at low tide. Formerly used as a bombing range, access to the island is prohibited because of the risk of unexploded ordnances. The area is still a gazetted Defence Practice Area, but is no longer in active use.