The Blue Mountains is a mountainous region in New South Wales, Australia. It borders on Sydney’s metropolitan area, its foothills starting approximately 50 kilometres west of the state capital. The area begins on the west side of the Nepean River and extends westward as far as Coxs River.

Consisting mainly of a sandstone plateau, the area is dissected by gorges up to 760 metres deep. The highest point of the range is 1,190 metres above sea level. 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 Mountains area includes the local government areas of the City of Blue Mountains, the City of Hawkesbury, the City of Lithgow and Oberon.

Official credit for crossing the Blue Mountains was eventually given to Gregory Blaxland, William Lawson and William Charles Wentworth. Following an exploratory trip in 1811, Blaxland, who wanted more grazing land, reasoned that the mountains could be crossed by following the ridges (thus creating the myth that the ridges were the easy way, when the easy way was in fact Coxs River). Accompanied by Lawson and Wentworth, he set out on May 11, 1813, and the party succeeded in crossing the mountains by May 31. They ventured as far as to what is now Mount Blaxland, just west of Cox’s River. On their return to Sydney, Governor Lachlan Macquarie granted them 1,000 acres (4.0 km2) of land each as a reward for their accomplishment.