Blias marplatensis sp. nov. (Copepoda, Chondracanthidae) is described from adult specimens recovered from the red searobin Prionotus nudigula Ginsburg, 1950, from Mar del Plata, Argentina. Females of the new species differ from those of B. prionoti, the unique and type species of the genus, by the presence of processes and knobs in both head and trunk, the shape of branches of antenna, by relative size and setation of legs 1 and 2 and the shape of caudal rami. Males of the new species also differ from B. prionoti in the armature of accessory antenna and by having shorter and lobate legs with different setation.