Mount Rerir was a mountain in Thargelion (a region of Beleriand) to the north of Lake Helevorn and one of the sources of the Gelion.

The land around the mountain belonged to Caranthir, who built a fortress on its western slopes. The fortress was destroyed during Dagor Bragollach. Elven forces probably withdrew from the area after the battle, and most definitely after Nirnaeth Arnoediad.