Onchidium is a genus of air-breathing sea slugs, shell-less marine pulmonate gastropod mollusks in the family Onchidiidae.

Some authors recognize only four species in the genus.

[4] Onchidium multinotatum, originally described within the genus in 1883, is regarded as a nomen dubium.

[5] SEM images of the radula can be found at Thompson, T.E.

"Scanning electron microscope studies of gastropod radulae".