The Tamar is a river in South West England, that forms most of the border between Devon (to the east) and Cornwall (to the west). It is one of several British rivers whose ancient name is assumed to be derived from a prehistoric river word apparently meaning "dark flowing" and which it shares with the River Thames.
The Tamar's source is less than 6 km (4 miles) from the north Cornish coast, but it flows southward. At its mouth, the Tamar flows into the Hamoaze before entering Plymouth Sound. Tributaries of the river include the rivers Inny, Ottery, Kensey and Lynher (or St Germans River) on the Cornish side, and the Deer and Tavy on the Devon side.