The South Downs is a range of chalk hills that extends for about 260 square miles (670 km²) across the south-eastern coastal counties of England from the Itchen Valley of Hampshire in the west to Beachy Head, near Eastbourne, East Sussex, in the east. It is bounded on its northern side by a steep escarpment, from whose crest there are extensive views northwards across the Weald. The South Downs National Park forms a much larger area than the chalk range of the South Downs and includes large parts of the Weald.

The South Downs is characterised by rolling chalk downland with close-cropped turf and dry valleys, and is recognised as one of the most important chalk landscapes in England. It is one of the four main areas of chalk downland in southern England.

The South Downs is relatively unpopulated, although in Sussex there has been large-scale urban encroachment onto the chalk downland by major seaside resorts, including most notably Brighton and Hove.

The South Downs has been inhabited since ancient times and at periods the area has supported a large population, particularly during Romano-British times. There is a rich heritage of historical features and archaeological remains, including defensive sites, burial mounds and field boundaries. Within the South Downs Environmentally Sensitive Area there are thirty-seven Sites of Special Scientific Interest, including large areas of chalk grassland.