Andustar was the western promontory of Númenor, separated from the Hyarnustar to the south by the wide Bay of Eldanna. The Andustar formed a very roughly rectangular region, about 240 kilometres west to east, and 160 kilometres north to south. To the east, it bordered the Forostar and Mittalmar regions, but in all other directions it was surrounded by the ocean.
The Andustar was a generally fertile region, except along its northern coasts, where the landscape became higher and rockier, and forested by firs. Its western shores were broken by three bays, with the northernmost of these being the Bay of Andúnië. On the high land around this bay, the Númenóreans built the port and city of Andúnië, whose Lords were to be the ancestors of the Kings of Arnor and Gondor.