Ruffia patagonica, a new genus and species of phreatogammarid amphipod, is described from a coastal cave on Madre de Dios Island (Chilean Patagonia). This is the first record of the family outside continental waters of New Zealand and New Caledonia. The new taxon displays some of the more distinctive features of the family, such as gnathopods with the palm margin lined with numerous unicuspid robust setae and with a denticulated inner margin of the dactylus; fifth pereiopod clearly smaller than pereiopod six and seven; “coxal gill” ( =  exopod) absent from the seventh pereiopod; oöstegites broadened; urosomites with posterodorsal robust setae; and third uropod equiramous with an elongated peduncle and with unsegmented cylindrical rami, the latter provided with numerous robust setae. In addition, the sternites of all pereionites, pleonites, and urosomites show a portion of integument delimited by hyaline frill that is presumably osmoregulatory in function, and is equivalent to the pair of sternal gills displayed by other phreatogammarids on several body somites. The new genus is unique among phreatogammarids in displaying sexual dimorphism in the third epimeral plate, in the protopod of the third pleopod, in the armature of the first uropod, and in the endopod of the second uropod. Even though Phreatogammaridae have been suggested to be related to the broadly distributed marine family, Melphidippidae, their restriction to continental waters of former Notogean territories points to vicariance by plate tectonics of a non-marine ancestor rather than to dispersal as the main mechanism leading to their current distribution pattern.