The Tamar is a river in south western England, that forms most of the border between Devon (to the east) and Cornwall (to the west). At its mouth, the Tamar flows into the Hamoaze where it joins with the River Lynher before entering Plymouth Sound. The river has some 20 road crossings, including the Tamar Bridge, a toll bridge on the A38 trunk road.

The Tamar's source is less than 4 miles (6 km) from the north Cornish coast, but it drains southward. North of the source the Cornish border heads to the sea along Marsland Water , making Cornwall nearly an island.

In a few places the border deviates from the river, leaving, for instance, the Devon village of Bridgerule on the 'Cornish' side. Curiously, the modern administrative border between Devon and Cornwall more closely follows the Tamar than the traditional border. Several villages north of Launceston which are west of the Tamar were actually in Devon until the 1960s.

The River Tavy is a tributary to the Tamar.

Related topics

• The Tamar River in Tasmania was named after the Tamar in Cornwall.
• Rivers of the United Kingdom