单颌鳗科为辐鳍鱼纲囊鳃鳗目的其中一科。

本科鱼眼小，体正常，无鳞片，头部前方骨变形。无侧线且无其他侧线孔特化组织，而仅以嗅觉器官代替。吻短，无上颔，舌固定，无锄骨，前后鼻孔靠近。鳃孔腹位；鳃被架骨无，体色为黑色，脊椎骨88-115。

Monognathus, or onejaw, is the only genus of the family Monognathidae of deep-sea, eel-like fishes. 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). The dorsal and anal fins lack bony supports, the pectoral fins are missing. The snout has a fang which is connected to glands.