Grammostola diminuta sp. nov. (Araneae: Theraphosidae) is described from northwestern Argentina; Grammostola vachoni Schiapelli and Gerschman 1961 is considered a senior synonym of Grammostola fossor syn. nov. Schmidt 2001 on the basis of material examined from the collection of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN-Ar). The geographic distribution of G. vachoni is extended on the basis of new records. Additionally, we present new diagnoses and new distributional data for Grammostola chalcothrix Chamberlin 1917, Grammostola inermis Mello-Leitão, 1941 and Grammostola pulchripes (Simon 1891).