The taxon studied was originally described by Löffler (1961) from Chile, who recognized the resemblance of these specimens with Eucyclops serrulatus (Fischer, 1851) and named it as a variety, E. serrulatus var. chilensis. Recently, Suárez-Morales & Walsh (2009) considered this taxon at the subspecies level, as E. serrulatus chilensis. In this contribution, and based on Argentinean specimens, the female is redescribed for the first time after its original description and the previously unknown male is described in full. The analysis of these specimens revealed a combination of characters that distinguish this taxon from E. serrulatus and from other more closely related species of the genus; based on this comparative analysis, this taxon merits a specific rank and should be named E. chilensis Suárez-Morales & Walsh, 2009. Diagnostic characters include the length/width ratio of the caudal rami and the extent of the spinulation along its outer margin, the length of the P5 medial spine relative to the length of the bearing segment, the relative lengths of P5 middle and outer setae, the length/width ratio of the third endopod segment of P4, and the ratio between the terminal spines. The very long P6 spine is also of diagnostic value in males. Additional diagnostic characters involving micro-ornamentation patterns of the antennal basipodite, and of the couplers and basipodites of the swimming legs, are also provided.