The River Arun's source is a series of small streams (known locally as ghylls or gills) in the St Leonard's Forest area, to the east of Horsham. It flows through Horsham to the west and at Nowhurst is joined by North River (aka River Ockle), whose source is the heights of Leith Hill and Holmbury Hill in Surrey.
It flows through Arundel and past the castle, and into the English Channel at Littlehampton. Its main tributary is the western River Rother. The Arun local government district in West Sussex is named after it. The river runs for approximately 41 km (25 miles) from source to the sea, and is one of the faster flowing in England[. The river is tidal as far inland as Pallingham Quay, 18.25 miles upstream from Littlehampton.