Blackadder Water (//) is a river in Berwickshire in the Scottish Borders area of Scotland, forming part of the River Tweed system. It reached 2.84m at Mouthbridge, which was its highest level ever recorded on Tuesday 22 October 2002 at 2:45pm.
Adder may be derived from Brittonic *ador, *edir or Old English edre, possible ancient hydronymic terms derived from an Indo-European formation meaning "a watercourse, a channel" (compare River Etherow). From the possible Old English derivation of ǣdre, "a vein" (Anglian ēdre), or *ǣdre, meaning "quickly", arise objections on the basis that these would have maintained the long initial vowel in English and Scots.
Rising in the Harecleugh Forest plantation just south of the Twin Law cairns, north of the village of Westruther, the headwaters of the Blackadder join with those of the Wedderlie, Edgar, and Fangrist Burns, before reaching the town of Greenlaw. The river proceeds in a meandering path through the Merse until it meets with the Langton Burn between the estates of Kimmerghame House and Wedderburn Castle. The Blackadder later becomes a tributary of the Whiteadder Water and joins with that river at the village of Allanton, where Blackadder House and its estate once stood.
- "Blackadder Water at Mouthbridge: River level and flood alerts". riverlevels.uk. Retrieved 2020-04-21.
- James, Alan G. "A Guide to the Place-Name Evidence - Guide to the Elements" (PDF). Scottish Place Name Society - The Brittonic Language in the Old North. Retrieved 25 October 2018.
- Johnston, James B (1892). Place-names of Scotland. Edinburgh, Scotland: D Douglas. p. 162. Retrieved 4 February 2019.
- RCAHMS record of Blackadder Water
- RCAHMS/Canmore record of Fogo Bridge
- Gazetteer for Scotland: Blackadder Water
- Streetmap of Blackadder Water