any of the marine gastropods that constitute the order Nudibranchia (subclass Opisthobranchia of the class Gastropoda). Nudibranchs possess a radular feeding organ, but they characteristically lack a shell, gills, and mantle cavity typical of other mollusks. The delicately coloured body has bizarre outgrowths, called cerata, which serve a defensive function, discharging nematocysts that the nudibranch has ingested from cnidarian prey. Cerata also function in gas exchange. Antenna-like organs (rhinophores) arise from the animal's head. Nudibranchs reach lengths of 43 cm (16 inches). About 40–50 percent of all opisthobranch species belong to this order.

Nudibranchs occur in the shallow waters of all the world's oceans, where they feed chiefly on other invertebrates, particularly sea anemones. Those of the family Tethyidae can swim. Among bottom creepers in cold northern seas is the bushy-backed sea slug (Dendronotus frondosus), named for its stalked, lacy cerata. Occurring worldwide in warm seas are the blue sea slug (Glaucus marina, or G. atlanticus) and the doridacean nudibranchs such as Doris and Glossodoris. See gastropod.

The term sea slug is sometimes used to refer to all members of the subclass Opisthobranchia (see opisthobranch).