A new species of caligid copepod, Caligus mulli n. sp., is described based on specimens collected from surmullet Mullus surmuletus Linnaeus from Atlantic waters off the Azores, Portugal, and from red mullet Mullus barbatus barbatus (Linnaeus) caught in the Mediterranean Sea, off the south-eastern coast of Turkey. Sixteen specimens (both sexes) were collected from the gill cavity of M. surmuletus, and one female was collected from the gill cavity of M. barbatus barbatus. The new species is described from both fish hosts. Detailed comparisons are presented with two closely related species, C. ocyurus Cressey, 1991 and C. biaculeatus Brian, 1914: the former can be distinguished by differences in shape of the dorsal cephalothoracic shield and postantennal process, while the latter differs in relative size of the distal spines on leg 4 and in the shape of the sternal furca. The new species can easily be distinguished from its congeners by a combination of the following additional characteristics: (1) the corpus of the female maxilliped bears a prominent projection proximally and the myxal area is ornamented with a hyaline flange and two sensillae, and (2) the male maxilliped has a conical proximal projection on the corpus and a bifid myxal process opposing the claw.