The River Plym is a river in Devon, England. Its source is some 450m (about 1500 ft) above sea level on Dartmoor, in an upland marshy area called Plym Head. From the upper reaches which contain antiquities and mining remains the river flows roughly southwest and enters the sea near (and gives its name) to the city of Plymouth, where the settlement was historically known as Sutton. Its tidal estuary is known as the Laira. It is approximately 30 km (about 20 mi) long.
The name Plym is thought to have its origins in Old English and means the 'plum tree', from a back-formation from the name of Plympton.
The Plym Valley Railway is a volunteer run railway which operates steam and diesel hauled train rides along a section of the Plym Valley from Marsh Mills station. The railway is currently extending its line to Plymbridge.