Glaucus atlanticus (common names sea swallow, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, a marine gastropod mollusk in the family Glaucidae. This is the only species in the genus Glaucus, but is closely related to Glaucilla marginata, which sometimes is included in Glaucus.
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.
Glaucus is a hermaphrodite, containing both male and female reproductive organs. Unlike most nudibranchs, which mate with their right sides facing, sea swallows mate with ventral sides facing. After mating, both animals produce egg strings.