Oplegnathus is currently the sole recognized genus in the knifejaw family (Oplegnathidae) of marine centrarchiform ray-finned fishes.

[5] The largest, the Cape knifejaw, can reach a maximum length around 90 cm (35 in).

Knifejaws have teeth fused into a parrot-like beak in adulthood.

[6] The earliest records of knifejaws are fossilized beaks, with attached teeth, known from middle Eocene-aged sediments of the La Meseta Formation of Antarctica.

Their early occurrence in Antarctica supports it having temperate climate during the Eocene, and that knifejaws had a wider distribution in the past than today.