Hemiphractus proboscideus adult females range from 57-66 mm long while the males are 43-50 mm. This species can be identified by a distinctive large triangular head, with a fleshy, sharply pointed snout and protruding tubercles on the upper eyelids (Duellman 2003). Frogs of the genus Hemiphractus are casque-headed, with large, angular ornamented skull bones; the skin is co-ossified to the cranial elements (Sheil et al. 2001). This species also has odontoids, fang-like outgrowths of the lower jaw which are superficially similar to teeth but not equivalent to true teeth (Trueb 1973; Shaw 1989a; Shaw 1989b). This frog has a flattened or depressed body. Vertebral neural spines form skin-covered projections on the back. Forearm tubercles are present and arrayed in neat horizontal rows. Heel spurs (calcars) are present. Dorsally, this species is brown or tan in color. The dorsal and lateral surfaces of the body and upper surfaces of the limbs have irregular green, brown, or gray stripes and spots. Ventrally, the coloration is brown with lighter tan or orange dots. The tongue and interior of the mouth are a striking yellow color (Duellman 2003; Bartlett and Bartlett 2003).

Embryos in the subfamily Hemiphractinae (marsupial frogs) are characterized by unique, membranous bell-shaped gills (Noble 1927; del Pino and Escobar 1981). These gills are derived from the branchial arches (Noble 1927; del Pino and Escobar 1981) and allow respiration during development in the egg (Mendelson et al., 2000). Hemiphractus proboscideus is a direct developer, and does not have a free-living larval stage (Trueb 1974; del Pino and Escobar 1981).