Pulmonate snail, Onchidium hongkongensis from muddy flat of Muan-gun was recorded as new to the Korean molluscan fauna. Shell-less body light-gray or reddish-gray in colour, oval shaped with numerous pustules on dorsal surface usually covered with mud. Black pair of tentacles with terminal eyes. Including the new record in this study, the family Onchidiidae 2 genera and 2 species in the Korean waters.