Three new species of Rockacestus Caira, Bueno & Jensen, 2021 were recovered from arhynchobatid skates taken between 37°S–55°S in the Magellanic Province, Southwestern Atlantic Ocean. Rockacestus blasi sp. nov. was found in Bathyraja macloviana (Norman, 1937), whereas Rockacestus magellanicus sp. nov., and Rockacestus ottavianoi sp. nov. were found in Bathyraja magellanica (Philippi, 1902). These species differ from their congeners in having a particular combination of anatomical features, including a moderate to highly folded bothridia, presence of a uteroduct, and a seminal receptacle. Cross-sections of mature proglottids were made for the first time in members of the genus. In addition, the microthrix pattern was described in detail, focusing on the distal bothridial surface, including the apical sucker and marginal loculi resulting in a common microthrix configuration with filitriches and small lanceolate, lingulate, and a particular kind of coniform spinitriches. The diagnosis of Rockacestus is revised to include several features exhibited by the new species. The distribution data of the species currently assigned to Rockacestus are compiled and updated. The finding of R. blasi sp. nov., R. magellanicus sp. nov., and R. ottavianoi sp. nov. not only increases the number of members of Rockacestus in the Magellanic Province in the Southwestern Atlantic from one to four but also expands our knowledge of phyllobothriideans and their association with the softnose skates of the genus Bathyraja Ishiyama, 1958 in the Southern Hemisphere.