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 within the Territory of Ashmore and Cartier Islands, an external territory of Australia. The land area of Cartier Island is about . It is located at , on the edge of the Sahul Shelf, about off the north west coast of Western Australia, south of the Indonesian island of Roti, and south-east of Ashmore Reef.