At maturity Glaucus atlanticus can be up to 3 centimetres (1.2 in) in length.[4] It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes on its head. It has a tapering body which is flattened, and has six appendages which branch out into rayed, finger-like cerata.[5]
The radula of this species bears serrated teeth