A new Pycnothelidae Chamberlin, 1917 species from Patagonian steppe, Chubut, Argentina is described and illustrated. Acanthogonatus messii Signorotto & Ferretti n. sp. differs from the remaining species of the genus by the male palpal bulb with a narrow basal portion and keel of embolus slightly-developed in conjuction with the presence of strong spines on ventral metatarsus I, females having spermathecae with short oval receptacles, and digitiform internal expansions arising from their base. This comprises a small species that construct burrows on sandy open environments with small shrubs. Results from phylogenetic analysis showed that is close to A. birabeni Goloboff, 1995 and A. chilechico Goloboff, 1995 inside the “patagonicus” group.