Ansonia leptopus is a common lowland stream toad. The individuals depicted on this page are from West Sarawak. Calling males sit on boulders and rocks on river banks at night. 

Adult males are 30–40 mm in snout-vent length, females are up to 65 mm. This species may be confused with Ansonia longidigita, which is a comparably large species often found in the same places. Ansonia leptopus has flat warts on its back. Larger flat warts are commonly present dorsolaterally. The nuptial pads in males are dark brown.

Ansonia leptopus tadpoles are very distinct. Black markings on a light background make the tadpole of this species unique. Only Ansonia longidigita larvae have such a contrasty pattern, yet in different shape. 

Tadpoles are small (< 20 mm). They are associated with submerged vegetation or leaf litter, and on rocks in stream sections with slow current. The oral sucker is less well-developed than in other Ansonia larvae.