This narrow-mouthed frog has a slightly ventrally shortened snout and a dark dorsal coloration. The deep purplish brown dorsum is accented by a well defined white line extending from the tip of the snout to the apex of the anal fold. The dorsal surfaces of the back and front legs are paler in comparison. The posterior surface of the thighs and legs, and the tarsus may have pale lines, but these are not very distinct. A pale line also exists on the throat. The surface of the venter has a uniform coloration of grayish tan. The skin is smooth and wart free. There is only the slightest amount of webbing between the toes and none between the fingers. This is the only species of Gastrophryne which has two metatarsal tubercles on each hind foot, rather than one (description from Legler 1964).
 
There has been discussion about the existence of subspecies of G. usta. Legler (1964) noted two subspecies: G. usta usta from Guerrero, Oxaca and Veracruz and G. usta gadovi, from Chiapas and Guatemala. He also mentioned that specimens from El Salvador have slight morphological differences from other members of the species, mainly in coloration. They are generally darker and more uniform in their dorsal coloration. Later publications by Duellman (1960) and others treat Gatrophryne usta as a single species.