Glaucus atlanticus (common names sea swallow, blue glaucus, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug.
The normal size of this species is up to 3 cm. It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes along the edge of its foot. It has a tapering body which is flattened and has six appendages which branch out into rayed cerata. Its radular teeth bear serrated teeth on their blades.