The spider genus Mazax O. P.-Cambridge, 1898 (Araneae: Corinnidae: Castianeirinae) is reported from South America for the first time. A new species, M. ramirezi sp. nov., is described and illustrated, and SEM images of the genus Mazax are presented here for the first time. The exclusive morphology of the secondary spermathecae (not oval and lung-shaped) indicates that the specimens described herein clearly belong to a new species of Mazax. An updated dichotomous key to the known species of the genus is provided.