The River Brathay is a short, fast-flowing, river in the Lake District of north-west England. It is no more than 2½ miles (4 km) long.
Draining the tarn of Elterwater the Brathay flows for about half a mile (0.8 km) in a south-easterly direction to Skelwith Force waterfalls where it descends 15 feet (4.6 m). Passing under the A593 road at Skelwith Bridge, the river continues in an easterly direction, passing over a weir, to the hamlet of Clappersgate. After another ¼ mile (400 m) it joins the River Rothay close to Croft Lodge south-west of Ambleside before flowing into the northern end of Windermere.
The stretch of the Brathay around Clappersgate is popular with canoeists.
For its entire length the River Brathay forms part of the northern boundary between the historic counties of Lancashire and Westmorland. Since local government re-organisation in 1974 the Brathay has been within the administrative county of Cumbria.