A new species of recluse spider, Loxosceles niedeguidonae sp. n., is described from the Parque Nacional Serra da Capivara, State of Piauí, Brazil. This is the first endemic species described from Brazilian semi-arid environment. The species is included in gaucho group of Gertsch (1967) due to its spermathecal shape and is considered close to L. chapadensis Bertani, Fukushima & Nagahama, 2010 by the unusual long male palpal tibia, a character not common for species of this group. An updated key for Loxosceles species of gaucho group is presented.