The hammerjaw, Omosudis lowii, is a small deep-sea aulopiform fish, found worldwide in tropical and temperate waters to 4,000 m (13,000 ft) depth.

It is the only representative of its family, Omosudidae (from the Greek omo, "shoulder", and Latin sudis, either "esox, fish of the Rhine" or "stake").

The body itself is scaleless and laterally compressed; it is covered in iridescent, silvery-gray guanine with the dark peritoneum peeking through in places.

The largest recorded hammerjaw measured 23 cm (9 inches) standard length (excluding the caudal fin).

They are carnivorous and feed on squid and other pelagic fish; in turn, hammerjaws are known prey of lancetfish and tuna.