Whiteadder Water is a river of Berwickshire, a tributary of the River Tweed. It flows through three counties; its upper waters are in East Lothian, amongst the Lammermuir Hills; it then passes most of its length in Berwickshire, then its lower reach enters the Liberties of Berwick in Northumberland, where it falls into the Tweed.
The stream wends its way southeast for approximately 3 miles where it joins with the Faseny Water to form the Whiteadder Reservoir formed in 1968, which supplies most of the towns of East Lothian and Berwickshire, with water (and the Cockenzie power station too).
The waters issuing from the reservoir soon cross into Berwickshire and the river runs alongside the B6355 road to Ellemford, where it is joined by the Dye Water and further on at Abbey St Bathans, the Monynut Water.
At Allanton the Whiteadder is joined by the Blackadder Water. The river proceeds by Foulden, Edrington, and Paxton, at which point it crosses into Northumberland, and soon joins the River Tweed, just north of East Ord, a suburb of Berwick-upon-Tweed.
For those with fishing permits, the Whiteadder provides recreational salmon and trout fishing. It is also the site of Ninewells, the childhood home of David Hume.