The River Esk, also called the Border Esk, is a river in Dumfries and Galloway, Scotland, that flows into the Solway Firth. It also flows for a small way through the English county of Cumbria before entering the Solway.

The river rises in the hills to the east of Moffat and its two main tributaries, the Black Esk and the White Esk, merge at the southern end of Castle O'er Forest. It flows south east through Eskdale past Langholm before merging with Liddel Water (which defines the border between Scotland and England. Before passing Longtown the river enters England and merges with the River Lyne and enters the Solway Firth near the mouth of the River Eden.