Australian island in the Indian Ocean, lying 12 miles (19 km) northwest of Fremantle (at the mouth of the Swan River, near Perth), Western Australia. A coastal limestone fragment, the island measures about 7 by 3 miles (11 by 5 km) and has sand dunes and several salt lakes. It was sighted in 1658 by a Dutch party under Samuel Volkerson, and in 1696 a Dutch sea captain, Willem de Vlamingh, gave the island its original name, Rottenest (meaning “rat's nest”), because the place appeared to be infested with large rats. These, in reality, were quokkas (quokka) (a kind of wallaby), rare marsupials whose existence is now protected by the island's status as a wildlife sanctuary.

First settled in the 1830s, Rottnest has served as a pilot station for Fremantle, an Aboriginal penal settlement (to 1850), a juvenile reformatory (1882–1906), and a World War II military base. It is now a resort for the Perth area and is accessible by sea and air.