river in the Scottish Borders council area of southeastern Scotland, flowing eastward for 97 miles (156 km) and forming for 17 miles (27 km) the border with England. For the last 2 miles (3 km) of its course, the Tweed flows through England before entering the North Sea at Berwick-upon-Tweed.

The river rises at Tweed's Wells and flows northeast in a flat-floored valley. Its upper tributaries include Talla Water, now dammed for Edinburgh's water supply, and Lyne Water. Flowing eastward through the Southern Uplands in a gorgelike valley to Melrose, the Tweed receives the Leithen, Gala, and the Ettrick-Yarrow system. From Melrose the Tweed flows through the broad agricultural basin of the Merse, and at Kelso it is joined by the Teviot (Teviot, River). The towns of the Tweed valley are woolen-manufacturing, market, and tourist towns, with several famous abbeys.