Microhyla petrigena is a lowland species of southeastern Sabah and central Sarawak. The species resembles M. perparvain both body proportions and coloration, but it is clearly the larger of the two species. M. petrigena is a stocky body with long hind limbs. Females can reach a snout–vent length of up to 18 mm, while males are somewhat smaller. A key distinguishing feature of M. petrigena is its darkly pigmented belly, which bears an irregular white blotch or marbling—absent in M. perparva. 

The skin is mildly tuberculate. The dorsal coloration consists of various shades of brown, often accompanied by dark shoulder markings and a pale band along the flanks. Larger dark markings at flanks, at anus and at thighs are outlined with light edges. A dark marking is central on the back, constricted just behind shoulder level and expanding posteriorly. A light stripe reaches from the eye to shoulder joint. Chest and belly mostly chocolate with conspicuous white blotches. The eardrum is invisible, covered under skin. The iris is finely stippled silver to brass on black background, with orange ring around the pupil.

Each hand has only three developed fingers, the first finger is reduced to a knob. Finger and toe tips are expanded and end bluntly. 

Breeding typically happens in quiet potholes of rocky streams (see image). The larvae appear very similar to those of M. perparva. The tail bears a brown cross-band in its distal third and ends in a short, narrow tip. Tadpoles reach approximately 16 mm in total length. The tadpoles are mid-water column suspension feeders. Like other members of the Microhylidae, they lack keratinized mouthparts. Their mouths are terminal, and the spiracle is located medially on the underside. The eyes are positioned far laterally on the head, a characteristic trait of the group.