The Arun (/ˈærən/) is a river in the English county of West Sussex. Its source is a series of small streams in the St Leonard's Forest area, to the east of Horsham. After flowing through Horsham to the west, it is joined by the North River at Nowhurst. Turning to the south, it is joined by its main tributary, the western River Rother, and continues through Arundel and past Arundel Castle, to join the English Channel at Littlehampton. The Arun local government district in West Sussex is named after it.