Whiteadder Water /ˈhwʌtədər/ is a river in East Lothian and Berwickshire, Scotland. It also flows for a very short distance through Northumberland before joining the River Tweed. In common with the headwaters of the Biel Water it rises on the low hillside of (122 m or 400 ft) in the Lammermuir Hills, just ESE of Whitecastle Hillfort and 3 km (1.9 mi) south-east of the village of Garvald.