 Eremoryzomys polius is a large, long-tailed rice rat that in color resembles some North American woodrats (Neotoma). The fur is grayish above and lighter below, where the hairs are gray at the bases but white at the tips. The external ears (pinnae) are short and the tail is dark above and light below. The hindfeet have well-developed ungual tufts (patches of hair) along the plantar margins and between all of the digits, a character shared only with Sooretamys angouya among oryzomyines. The squamae, small structures resembling scales that cover the soles of the hindfeet in many oryzomyines, are well developed. The claw of the first digit extends nearly to the end of the first phalanx of the second toe and the claw of the fifth toe extends slightly beyond the first phalanx of the fourth toe. As in most oryzomyines, the female has eight mammae. Head and body length is 138 to 164 mm (5.4 to 6.5 in). In Osgood's original two specimens, an old female and an adult female, tail length is 188 and 180 mm (7.4 and 7.1 in), respectively; hindfoot length is 30 and 30 mm (1.2 and 1.2 in); and greatest skull length is 37 and 34.7 mm (1.46 and 1.37 in). Eremoryzomys polius has 12 thoracic, 7 or 8 lumbar, and 35 or 36 caudal vertebrae; the presence of 12 thoracic vertebrae is a putative synapomorphy of Oryzomyini.
