Glaucus marginatus is a species of small, floating, blue sea slug; a pelagic (open-ocean) aeolid nudibranch; a marine opisthobranch gastropod mollusc in the family Glaucidae.

[4] This nudibranch is dark blue, and in many ways it resembles a smaller version of Glaucus atlanticus.

However, in this species the cerata are arranged in a single row in each arch.

[citation needed] While they do not usually inhabit coastal regions, hundreds of the creatures were observed washing up on one of the Sydney North Shore beaches, near Long Reef, in February 2021.

[4] These small nudibranches float upside down on the surface tension in temperate and tropical seas.