Awash Mastomys is known from Ethiopia's Upper Awash. A reddish band separates its dark back from its gray belly.
Its back is blackish and its flanks grayish-rufous. A conspicuous rufous band separates them from the gray belly; the cheeks are yellowish-rufous and the backs of the hands and feet are white.
The paper describes riverbank Acacia–Commiphora thorn scrub with tall grass, beside agricultural land. It treats the species as an outdoor occupant rather than a house-dwelling mouse.
An extinction-risk assessment has not been added yet.