The River Arun is a river in the English county of West Sussex. At 37 miles (60 km) long, it is the longest river entirely in Sussex and one of the longest starting in Sussex after the River Medway, River Wey and River Mole. It is one of the faster flowing rivers in England, and is tidal as far inland as Pallingham Quay, 25.5 miles (41.0 km) upstream from the sea at Littlehampton.
The first major improvements to the river were made between the 1540s and the 1570s, when Arundel became a port, and navigation up to Pallingham was improved, but barges had difficulty negotiating the flash locks that were installed. With the coming of the railways and changes in coastal shipping, Littlehampton superseded Arundel as the port of the Arun.