The Sierra del Tontal chinchilla rat is a smallish, rat-like animal, with a head-body length of about , a tail about long, and an average adult body weight of . It has drab greyish fur, which is paler on the underside, and has a faint stripe of darker fur running down the middle of the back and along the upper surface of the tail. The upper surfaces of the feet are covered with white fur, and there is also a distinct patch of white fur on the chest, covering a scent gland. The lower surfaces of the feet have thick pads with leathery tubercles, which help the animal gain traction on bare rock.