The river's source is around 450 metres (1,480 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 past clay workings at Shaugh Prior to Dewerstone, where it meets the River Meavy.

 The course then changes to run southwards, between Plymouth and Plympton.

 Its tidal estuary is known as the Laira and the final part is called Cattewater, which is navigable and leads towards Plymouth Sound.

It is approximately 20 miles (32 km) long.