Ruy (Bulgarian: Руй) or Ruj (Serbian Cyrillic: Руј), is a mountain range in the Kraishte region on the border of western Bulgaria and southeastern Serbia.

On a clear day may be observed the ranges of Rila and Vitosha in Bulgaria and Veliki Strešer in Serbia.

To the north, northwest and west it extends to the valley of the river Blatanica (a left tributary of the Erma), and to the southwest through a saddle at an altitude of 1243 m, it is connected to Miloslavska planina.

It is covered with a mixed deciduous forest of beech, oak and hornbeam, as well as extensive pastures at the highest altitudes.

[1] In the easternmost part of the mountain range is situated the Tran Gorge, an important natural landmark and one of the 100 Tourist Sites of Bulgaria.