Avahi unicolor is one of the smallest woolly lemurs and consequently among the smallest members of the family Indriidae. It has a head-body length of 23–31.0 cm, a tail length of 26.5–30.3 cm, a total length of 57.5–66.0 cm, and a weight of 700 g–1.0 kg (E. E. Louis Jr., pers. obs.). According to Thalmann and Geissmann (2000), the coat is woolly and the dorsal color is a sandy brownish-gray. The tail may be a darker grayish-brown or more reddish-brown, although the base of the tail tends to be lighter, even cream-colored. There is a triangular beige- or cream-colored pygal patch. The ventral coat is thinner than the dorsal coat and a lighter gray. The face is only slightly paler than the head and back, with short and straight hair, giving the impression of a facial ring or mask. The snout is hairless and black, the fur at the corners of the mouth whitish, and the eyes maroon.
In the field, Avahi unicolor is most likely to be confused with Lepilemur and, to a lesser extent, with Cheirogaleus, but it is not sympatric with other Avahi. It is significantly larger than Cheirogaleus and quite different in its postural and locomotor behavior, and can usually be distinguished from Lepilemur by its less conspicuous ears, denser, curlier coat, and obvious white thigh patches. It can be distinguished from Avahi occidentalis by the latter’s distinct white facial mask and dark eye-rings, and from Avahi cleesei, which has a dark chevron pattern on its forehead (Thalmann and Geissmann, 2000).