Monkfish (or Headfish) is the English name of a number of types of fish in the northwest Atlantic, most notably the species of the anglerfish genus Lophius and the angelshark genus Squatina. The term is also occasionally used for a European sea monster more often called a sea monk.

Monkfish is the most common English name for the genus Lophius in the northwest Atlantic but goosefish is used as the equivalent term on the eastern coast of North America. Lophius has three long filaments sprouting from the middle of the head; these are the detached and modified three first spines of the anterior dorsal fin. As in most anglerfish species, the longest filament is the first (illicium), which terminates in an irregular growth of flesh, the esca. This modified fin ray is movable in all directions. This esca is used as a lure to attract other fishes, which monkfish then typically swallow whole. Experiments have shown, however, that whether the prey has been attracted to the lure or not is not strictly relevant, as the action of the jaws is an automatic reflex triggered by contact with the esca.

It grows to a length of more than 1.5 m (5 ft); specimens of 1 m (3 ft) are common. The largest recorded specimen caught weighed 99.4 kg (219 lbs).