A new species of Oligoxystre, Oligoxystre diamantinensis n. sp., is described from Brazil. Male can be distinguished from all other Oligoxystre species by the male palpal bulb lacking a small subapical keel on the embolus in conjunction with the embolus length (less than 2.5 times the tegulum length) and by the tibial spur being inserted in a perpendicular angle in relation to the tibia axis. Female can be distinguished by the spermathecae being much more longer than wide, lacking lateral lobes anda large terminal lobe with five smaller lobes around it. Additionally, males and females can be distinguished by the general blue metallic color pattern contrasting with the reddish setae on the abdomen. A key for all described Oligoxystre species is presented. The cladistic relationship of O. diamantinensis sp. n. with the other Oligoxystre species is discussed and data on its habitat is provided.