[1] Adder may be derived from Brittonic *ador, *edir or Old English edre,[2] possible ancient hydronymic terms derived from an Indo-European formation meaning "a watercourse, a channel" (compare River Etherow).

[2] Also suggested is derivation from Gaelic fad dûr,[3] meaning "long water".

[3] 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 Whiteadder then crosses the English Border below Edrington where it joins the Tweed which empties into the North Sea at Berwick-upon-Tweed.