Monognathus, or onejaw, is the only genus of the family Monognathidae of deep-sea eels.

The name comes from the Greek monos meaning "one" and gnathos meaning "jaw", a reference to the large mouth in comparison with the rest of the fish, and also the absence of an upper jaw (maxilla and premaxilla bones are absent).

[3] The dorsal and anal fins lack bony supports and the pectoral fins are missing.

The snout has a fang connected to glands, which are venomous in adults.

[4] These eels are virtually blind, with rudimentary eyes and small olfactory organs, although the male olfactory organs are enlarged, perhaps suggesting a method of "sniffing out" a mate.