Leptomantis belalongensis was described by Dehling and Grafe (2008) from lowland rainforests near Kuala Belalong Field Studies Centre, Temburong District, Brunei Darussalam (Borneo). The altitudinal range is not well established but the species is believed to be restricted to the lowlands. 

The dorsum is grey to light brown, more or less regularly speckled with small dark brown irregularly shaped spots. There are irregularly shaped sky blue blotches on flanks and anterior surface of the thigh. White tubercles but no dermal flap along forearm. The iris is ruby-colored around pupil, yellowish peripherally. Size is usually less than 38 mm in females and up to 31 mm in males.

Dehling and Grafe reported: "Males and females were found on vegetation next to small, fast-flowing creeks at heights between one and three meters above the ground. Males were also heard calling from the crown region of small trees that were up to 10 m high."

This species is very similar to L. gauni but lacks (among other things) the conspicuous conical tubercles above the eye ususally present in L. gauni (see description of L. gauni) Recently the validity of this species has been questioned because of small genetic distances from L. gauni populations. Its species status needs further research.