The once-married underwing (Catocala unijuga) is a moth of the Erebidae family. It is found from Newfoundland west to south central British Columbia, south to Kentucky and Missouri in the east, Colorado and Utah in the west. The wingspan is 68–82 mm. Adults are on wing from July to September in one generation depending on the location. The larvae feed on Populus tremuloides, Populus nigra and Salix species.