A new species of cyclopid copepod, Acanthocyclops magistridussarti sp. nov., is described herein and attributed to the Acanthocyclops vernalis-group.Within this species-group, Acanthocyclops magistridussarti sp. nov. shows the closest affinities with Acanthocyclops cf. orientalis sensu Petkovski, 1975, collected from ground waters in Macedonia and Bosnia, and Acanthocyclops orientalis sensu Lee et al., 2007 from Korean caves, characterized by long setae of the distal segment of the leg 4 endopod. The new species can be distinguished from all other species of the group by the peculiar ornamentation of the antennary basis, which, however, shows a surprising variability in the pattern of presence/absence of the group of spinules proximal to the insertion of the exopodal seta, of the oblique row of spinules inserted in the middle of the first 1/3 of the segment on the frontal surface, and of the oblique row of short spinules below the two distal setae. This latter row of spinules has never been recorded in the Acanthocyclops vernalis-group. Notwithstanding such variability, one diagnostic microcharacter showed to be constant: the group of six long spinules near the outer margin on the caudal side of antennary basis, which are inserted to form a tuft rather than a row, as is usual in the Acanthocyclops of the vernalis-group. The new species is so far known only from groundwater habitats, having been collected in caves, hyporheic habitats, and springs in the Apennines (peninsular Italy).