This species is a simple, solitary ascidian, with a tough leathery outer covering or tunic. The body is ovoid in shape, measures up to 5 cm in length, and attaches to the substrate near its base. Oral siphon is positioned anteriorly; atrial siphon is located midcenter, often projecting at a right angle. Tips of both siphons are conspicuously ringed in alternating white and dark red bands. Internally, the siphons possess distinct, short siphonal spines, with each spine characterized by a rounded, serrated edge. Overall, the tunic is tannish-brown with bright orange or red markings, but often covered with very short hairs and debris.
Microcosmus squamiger is most similar to M. exasperatus Heller, 1878, a species that presently is not found in the Northeast Pacific.
Microcosmus squamiger has been introduced to the Northeast Pacific, with confirmed records in southern California and Baja California Mexico. In addition, this species has been introduced to other locations around the world, including the Northeast Atlantic, the Mediterranean, and South Africa. It is native to southern Australia.