The River Tweed, or Tweed Water, (Scottish Gaelic: Abhainn Thuaidh, Scots: Watter o Tweid) is 97 miles (156 km) long and flows primarily through the scenic Borders region of Scotland, and eastwards from the settlements on opposing banks of Birgham and Carham forms the historic boundary between Scotland and England. It rises in the Lowther Hills at Tweed's Well near where the Clyde, draining northwest, and the Annan draining south also rise. "Annan, Tweed and Clyde rise oot the ae hillside" is a saying from the Border region. It drains the entire Borders region. Its lower reaches are near Berwick-upon-Tweed. The Tweed is one of the great salmon rivers of Britain and the only river in England where an Environment Agency rod licence is not required for angling.
The name of the river comes from the Gaelic word for north, "thuaidh" which was later Anglicized into Tweed. Tweed (cloth) derives its name from its association with the River Tweed.