Avahi occidentalis is one of the smallest woolly lemurs and consequently among the smallest members of the family Indriidae. It has a head-body length of 26.9–30.3 cm, a tail length of 30.7–37.7 cm, a total length of 57.6–68.2 cm, and a weight of 800–1,100 g (Bauchot and Stephan, 1966; Thalmann and Geissmann, 2000; Thalmann, 2001; Zaramody et al. 2006; Lei et al., 2008). It is lighter in color than A. laniger, with the dense, tightly curled fur of the back a light to medium gray, sometimes flecked with brown or olive, becoming paler towards the rear. Typically, the tail is gray as well, but can sometimes be reddish. The face, throat and cheeks are pale, not brown as in A. laniger.
Avahi occidentalis is most likely to be confused with the other species of western woolly lemurs, with Lepilemur and, to a lesser extent, with Cheirogaleus medius. It can be distinguished from Avahi cleesei and Avahi unicolor by its contrasting white facial mask and dark eye-rings (Thalmann and Geissmann, 2000). 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. As with other woolly lemur species individuals frequently huddle together with other family members, a behavior very rare in other nocturnal species of comparable size.