The River Dun is a river flowing through the counties of Wiltshire and Berkshire in England. It rises near Great Bedwyn, Wiltshire and flows north-east into Berkshire, where it joins the River Kennet at Hungerford, ultimately draining to the North Sea via the Thames.
The Dun valley is an important transport route through the high chalklands between the London Basin to the east and the Vale of Pewsey to the west. It is the route by which the Kennet and Avon Canal (linking London and Bristol) enters the Thames basin from the Vale of Pewsey, crossing the watershed with the aid of the Bruce Tunnel and Crofton Pumping Station. The later Reading to Plymouth railway linking London and the south-west also uses the valley.