Onchidella celtica is a species of air-breathing sea slug, a shell-less marine pulmonate gastropod mollusk in the family Onchidiidae.

[3] Onchidella celtica is up to 12mm long and its oval body has a deep olive to black colour.

[4] This species occurs locally on the East Atlantic coastline, from western Scotland, western England and the Channel Islands, south to Spain and the Azores, including: This air-breathing sea slug lives in the intertidal zone on rocky shorelines.

[4] It can absorb small amounts of dioxygen through its mantle, allowing it to stay submerged during high tide.

[5] This species is hermaphrodite, using reciprocal mating to produce between 60 and 100 tubular eggs gathered as a jelly mass.