Big-eared opossums closely resemble another Neotropical marsupial, common opossums (Didelphis marsupialis). In fact, this species was once considered a subspecies of Didelphis marsupialis. Big-eared opossums have prominent facial markings and a conspicuous black line down the centre of their forehead. Their ears are naked and black. Their fur is dirty yellow, with black or grey tips. Big-eared opossums have long, prehensile tails that are furred at the base. The fur at the base of their tail is about as long as their hind legs and is at least half black and half white; the black portion is sometimes longer. In contrast, common opossums do not have as much fur on the base of their tail and they usually have a shorter black portion. Aside from geographic location, this is one characteristic that can be used to distinguish the species.

Male big-eared opossums tend to be larger than females. Adult males range from 1,500 to 1,880 grams during the reproductive season. Females can weigh anywhere from 1,000 to 1,300 grams in the reproductive season.