The South Downs is one of the two areas of chalk downland in southern England. The North & South Downs run parallel to each other, and would once have formed part of the same dome-shaped chalk outcrop. (See Southern England Chalk Formation.) Erosion, however, has removed the chalk between the two ridges, forming an area called the Weald.

The South Downs extend about 70 miles (100 km) through East Sussex, West Sussex and part of Hampshire. Towns and villages include Eastbourne with its 164 metre high headland Beachy Head, Lewes, Brighton and Hove, Shoreham-by-Sea, the villages of Washington, Arundel and Midhurst.

The most famous cliffs of the South Downs, apart from Beachy Head, are the Seven Sisters, between Eastbourne and Seaford. The highest point on the South Downs is Butser Hill , just south of Petersfield, Hampshire.

Much of the South Downs have been designated Areas of Outstanding Natural Beauty in England.

A national park in the South Downs is proposed, and received support from the government in September 1999. As of 2004, a public inquiry is being held to decide the boundaries of the proposed national park. The designation process is expected to take another two to three years.