The Blue Mountains is a Sout Wales, Australia. It borders on Sydney's metropolitan area, its foothills starting about 50 kilometres (31 mi) west of the state capital. The public's understanding of the extent of the Blue Mountains is varied, as it forms only part of an extensive mountainous area associated with the Great Dividing Range. Officially the Blue Mountains is bounded by the Nepean and Hawkesbury Rivers in the east, the Coxs River and lake Burragorang to the west and south, and the Wolganand Colo Rivers to the north. Geologically, it is situated in the central part of the Sydney Basin.

The Blue Mountains are a dissected plateau carved in sandstone bedrock. They are now a series of ridge lines separated by gorges up to 760 metres (2, 490 ft) deep. The highest point in the Blue Mountains, as it is now defined, is an unnamed point with an elevation of 1,189 m (3,901 ft) seve kilometres north-east of Lithgow. However, the highest point in the broader region once considered to be the Blue Mountains is Mount Bindo, elevation 1,362 m (4,469 ft). A large part of the Blue Mountains is incorporated into the Greater Blue Mountains Area World Heritage Site, consisting of seven national park areas and a conservation reserve.

The Blue Mountain area includes the local government areas of the City of Blue Mountains, the City of Hawkesbury, the City of Lithgow and Oberon Shire.