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A medium-size (3.8-4.2 cm wingspan) moth with dark bluish ash grey forewings and brown hindwings. Forewings are dark slate grey with a dull metallic silvery cast, finely mottled by darker black and lighter grey scales. The stigma is a green-gold V with a prominent distal satellite spot. Hindwings are rich dark brown shading to a wide sooty brown terminal band. The antennae are simple and the sexes are alike. Very similar to the much rarer S. selecta, which averages a bit smaller and has paler brown hindwings that contrasts more with the dark terminal band. Genitalia of lighter specimens should be examined for positive identification. The valve of the male is broad and rounded in viridisigma, with a small finger-like clasper extending less than half way across the valve and two cornuti in the vesica (a tapered valve in selecta, with a larger, triangular clasper extending to the dorsal margin of the valve, and a single cornutus). Females of selecta have a large sclerotized caudal process of the ostium bursa, which is lacking in viridisigma.

The larvae feed on a variety of coniferous trees, including spruce (Picea), fir (Abies), Douglas-fir (Pseudotsuga), pine (Pinus), tamarack (Larix), and cedar (Thuja). However, spruces and firs are by far the most frequently recorded hosts (Prentice, 1962).

Newfoundland west across the boreal forest to Alaska, south in the east to Massachusetts, New York and Michigan and in the west to southern Oregon. In the Rocky Mountains they occur south to Arizona and New Mexico. In Alberta, viridisigma is found throughout the Boreal forest, foothills and mountain regions, and in the Cypress Hills.