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). A large part of the valley of the Tamar is protected as the Tamar Valley National Landscape (an Area of Outstanding Natural Beauty), and some is included in the Cornwall and West Devon Mining Landscape (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 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 in 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.