The Penitent Underwing (Catocala piatrix) is a noctuoid moth form North America. It is placed in subfamily Catocalinae, either of the family Noctuidae, or – if the Noctuidae are circumscribed more strictly – of family Erebidae. Within the Catocalinae, it belongs to tribe Catocalini and – if the Noctuidae are circumscribed widely – subtribe Catocalina.

The wingspan of adults is about 70-80 mm. The forewings are dar brownish grey, darkest at the base, and with an oblique lighter band extending from the midwing to the leading edge near the wing base. The hindwings are yellow-orange with dark hairs at the base (though these are not especially prominent) and two concentric black bands. The yellow zone separating the black bands is wide, with a relatively smooth edge; the outer edge of the hindwings is light yellow with some black bars extending from the outer band. As typical for hickory-feeding Catocala, both foreleg and hindleg tibiae of this species are spiny, and the tarsi carry four rows of irregular rows of spines each.

The moths flies from July to November depending on the location. The larvae feed on Ash, Butternut, Hickory, Pecan, Persimmon, and Walnut trees.