Microhyla petrigena is similar to M. perparva in body proportions and coloration, but is clearly larger than M. perparva. Also, petrigena has a dark belly with an irregular white blotch. Females reach 18 mm snout-vent length, males are smaller.

The frogs are colored in shades of brown above with dark markings on the shoulder and a light band on the flank. There are only 3 fingers on the hand. Toe tips are expanded.

Tadpoles were described (verbally, no image) in Inger & Steubing 2005 and appear to be similar to those of M. perparva. The tail bears a brown cross band in the distal third. The tail terminates in a short, narrow tip.

Tadpoles are mid water column suspension feeders. They lack keratinized mouthparts. The mouth is terminal. The spiraculum is located in a medial position on the venter.  The eyes of the tadpole are far lateral on the head.