The hands, feet, and whiskers are black. The giant tree rat has the usual murid body form and large, broad, heavy feet. The pollex has a short nail, and the other digits have considerably enlarged and slightly curved claws. The muzzle is short, and the skull is heavy and thick. The moderately broad incisors are ungrooved and project slightly forward.
Females have six mammae.
It seems to be mainly arboreal. Its lair is usually in a hollow tree, often at a considerable height, but is sometimes on the ground. It is entirely vegetarian, feeding largely on shoots such as those of climbing bamboo. In some areas the local people said that there is a second kind of Mallomys that is more terrestrial, is found in open country as well as forest, and lives in holes in the ground. This claim seems to have been borne out by the report of Flannery et al.