Among the polychaetes, the family Cirratulidae is one of the most challenging taxonomically because most of the currently used diagnostic characters change during ontogeny. It is therefore necessary to use a combination of characters to make an accurate identification. The present work is the first of a planned revision of the genus Cirratulus from coastal zones of Argentina. The examined material came from the two largest natural history museums of the country, and showed several morphotypes. Some specimens corresponded to described species, but others were undoubtedly undescribed. Among the previously known Cirratulidae, Cirratulus jucundus (Kinberg, 1866) and Cirratulus patagonicus (Kinberg, 1866), formerly distributed for Antarctica are now also present in coastal continental areas of Argentina. The new species Cirratulus mianzanii sp. nov. is described from subtidal areas off Peninsula Valdés, Patagonia. This species is distinguished from its congeners by the segmental origin of both tentacular filaments and first pair of branchiae in the junction between the peristomium and first chaetiger. Branchiae arise some distance from the notopodium but not at the dorsal midline of the body. Ventral groove present along the body, with a fine dark midline. Capillary chaetae are serrate seen under an optical microscope, and the segmental origin of neuro- and notopodial spines is different from other species of the genus.