Rhacophorus penanorum is currently known as endemic of Gunung Mulu. It has been described only recently by Dehling (2008) based on specimens of the Tapin Valley, at approx. 1.650 m asl.
Size of adults: approx. 33-34 mm in males. Female size is unkown.
The known specimens were bright green above during the night but can change to a more brown hue during the day (see photo). The flanks bear dark irregular spots on yellow or cream background. The iris is can be ruby colored, but has been observed to become paler during the day. The iris margin is grey.
The most similar known species is Rhacophorus anguliostris, also a highland species. It differs from penanorum in having a blue peripheral upper iris coloration, brown dorsal color with dark blotches, an interorbital band, and (in most but not all specimens) white blotches below the eyes.
Tadpoles have not been assigned unequivocally to this species yet.