Garden Island is a slender island about 10 kilometres (6 mi) long and 1.5 kilometres (0.9 mi) wide, lying about 5 kilometres (3 mi) off the Western Australian coast, to which it is now linked by a man-made causeway.
Like Rottnest Island and Carnac Island, it is a limestone outcrop covered by a thin layer of sand accumulated during an era of lowered sea levels. The Noongar Indigenous Australians tell of walking to these islands in their Dreamtime.
At the end of the last glacial period, the sea level rose, cutting the island off from the mainland. For the last seven thousand years the island has existed in relative isolation.
The Royal Australian Navy's largest fleet base, Fleet Base West, also called HMAS Stirling, are on the shores of Careening Bay, on the southeastern section of Garden Island, facing Cockburn Sound.
Garden Island is home to a tammar wallaby population.