The species name aurantium is derived from Latin, meaning “orange,” referring to the orange coloration of the groin and the inner and lower surfaces of the thighs. The type locality is Mendolong, Sipitang District, Sabah (Inger 1989).

Malkmus & Riede (1996b) noted the existence of a second, similar taxon that is morphologically very close to Philautus aurantium Inger, 1989 but exhibits a markedly different call. This taxon may represent a distinct species, Philautus gunungensis, which has also been reported only from Gunung Kinabalu. If valid, P. gunungensis would be difficult to distinguish in the field from P. aurantium. Due to the lack of confirmatory evidence from our own specimens, we refer here only to P. aurantium, but we note the possible existence of P. gunungensis. Further research employing detailed genetic analyses would be highly desirable.

Philautus aurantium is a small frog, measuring 25–26 mm in snout–vent length (SVL). The snout is broadly rounded in dorsal view and gently rounded in profile. The nostrils are situated on slightly raised prominences, and the canthus rostralis is smoothly rounded. The digits and toes terminate in expanded discs. A distinct, oval inner metatarsal tubercle is present, while an outer one is absent. Finger webbing is rudimentary, and toe webbing is moderate. The tympanum is indistinct and lies below an almost straight supratympanic fold. The dorsal skin is smooth, with occasional small tubercles or slight wrinkling above the eyes. The chest and belly are coarsely granular.

Dorsal coloration is sandy, with scattered dark brown to black spots, particularly along the posterior flanks. The dorsal midsection, from the nostrils to between the eyes and along the back, may be slightly darker, interrupted by a pale cross-line between the eyes. Round black spots are scattered along the flanks, on the posterior dorsum, and on parts of the legs. The groin and ventral surfaces of the thighs are orange. The pupil is horizontally elliptical, and the iris is beige to golden with reddish venation.

This species inhabits primary montane and submontane forests at elevations of 750–1000 m a.s.l. in Sarawak and western Sabah. Males call from saplings and shrubs, typically spaced apart from one another. The reproductive mode remains unknown.