Coordinates: 55°45′14″N 2°33′11″W / 55.754°N 2.553°W
Blackadder Water () is a river in Berwickshire in the Scottish Borders area of Scotland, forming part of the River Tweed system.
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.
The Whiteadder then crosses the English Border below Edrington where it joins the Tweed which empties into the North sea at Berwick-upon-Tweed.