River Tamar

River Tamar (Dowr Tamar )
The Tamar near Bohetherick
Country	England
Regions	Cornwall, Devon
Source	Woolley Moor, Morwenstow parish
- location	50°55′25″N 4°27′44″W / 50.9235°N 4.4622°W / 50.9235; -4.4622, Cornwall
- elevation	206 m (676 ft)
Mouth	Hamoaze
- location	Plymouth Sound, English Channel
- coordinates	50°21′30″N 4°10′0″W / 50.35833°N 4.16667°W / 50.35833; -4.16667Coordinates: 50°21′30″N 4°10′0″W / 50.35833°N 4.16667°W / 50.35833; -4.16667
Length	98 km (61 mi)
Discharge	for Gunnislake
- average	22.55 m3/s (796 cu ft/s)
- max	714.6 m3/s (25,236 cu ft/s) 28 December 1979
- min	0.58 m3/s (20 cu ft/s) 23 August 1976
Discharge elsewhere (average)
- Crowford Bridge	2.34 m3/s (83 cu ft/s)
Map of the Tamar catchment

The Tamar (/ˈteɪmɑːr/; Cornish: Dowr Tamar) is a river in south west England, that forms most of the border between Devon (to the east) and Cornwall (to the west). The area is a World Heritage Site due to its historic mining activities.

The Tamar's source is less than 6 km (3.7 mi) from the north Cornish coast, but it flows southward and its course runs across the peninsula to the south coast. The total length of the river is 61 miles (98 km). At its mouth, the Tamar flows into the Hamoaze before entering Plymouth Sound, a bay of the English Channel. 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.

The name Tamar (or Tamare) was mentioned by Ptolemy in the second century in his Geography. The name is said to mean "Great Water." The Tamar is one of several British rivers whose ancient name is assumed by some to be derived from a prehistoric river word apparently meaning "dark flowing" and which it shares with the River Thames.

The seventh century Ravenna Cosmography mentions a Roman settlement named Tamaris, but it is unclear which of the towns along the Tamar this refers to. Plymouth, Launceston and the Roman fort at have been variously suggested.