The Simpson Desert is a large area of dry, red sandy plain and dunes in Northern Territory, South Australia and Queensland in central Australia. It is the fourth largest Australian desert, with an area of 176,500 km2 (68,100 sq mi) and is the world’s largest sand dune desert.

The desert is underlain by the Great Artesian Basin, one of the largest inland drainage areas in the world. Water from the basin rises to the surface at numerous natural springs, including Dalhousie Springs, and at bores drilled along stock routes, or during petroleum exploration. As a result of exploitation by such bores, the flow of water to springs has been steadily decreasing in recent years. It is also part of the Lake Eyre basin.

The Simpson Desert is an erg which contains the world’s longest parallel sand dunes. These north-south oriented dunes are static, held in position by vegetation. They vary in height from 3 metres in the west to around 30 metres on the eastern side. The largest dune, Nappanerica, is 40 metres in height.