R. onca has a snout-vent length between 44 and 87 mm, and is distinquished from similar species of the Rana pipiens complex by its short, indistinct, dorsolateral folds that extend 1/2 to 3/4 down the dorsum, generally shortened legs, an incomplete supralabial stripe, upper surfaces of the thighs usually spotted and not barred, and males having enlarged tympana, paired vocal sacs, and no vestigial oviducts. It has a dorsum which is brown, gray, or greenish above, with discrete greenish-brown spots that are usually reduced or faded on the front of the body. These dorsal spots are indefinitely bordered and are usually present on the upper surfaces of the thighs. Usually, the venter is whitish, and the throat has dark mottling. The groin and the undersides of the hind limbs are yellow to yellow-orange.
This is a minor success story. The species was long thought to be extinct or near extinction. Although extant populations have been found, they exist in isolated springs and creeks in desert environments, near reservoirs and recreation sites in an area that has rapidly urbanized. Accordingly, the species must be considered to be highly vulnerable. Jaeger et al. (2001) recommend development of a conservation management plan for the few remnant populations; such a conservation plan must prioritize the identification of habitat requirements and the reclamation of habitats necessary to maintain population viability.