A new species of ant-resembling sac spider of the subfamily Castianeirinae, Grismadox elsneri sp. nov., is described from the Sub-Andean area of the Bolivian orocline. The species was collected from savanna grass along the edges of the Chiquitano forest and is a putative mimic of the carpenter ants Camponotus cf. crassus or C. cf. blandus.