Leptomantis belalongensis is a rencently discovered new species from Brunei lowland rainforests. 

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."

Recently the validity of this species has been questioned because of small genetic distances from L. gauni populations.