Malaxis irmae, a new orchid species from the Paraná and Uruguay river basins in northeast Argentina and Uruguay, is described and illustrated. It is similar in size and overall floral morphology to Malaxis cipoensis, a species endemic to upland rocky fields on the Espinhaço range in Southeastern Brazil, which is its closest relative according to a cladistics analysis of nuclear (ITS) and plastid (matK) DNA sequences presented here. However, M. irmae is distinguished from M. cipoensis by inhabiting lowland marshy grasslands, possessing 3−5 long-petiolate leaves per shoot (vs. 2 shortly petiolate leaves), cylindrical raceme (vs. corymbose), pale green flowers (vs. green-orange flowers) and less prominent basal labellum lobules. Malaxis irmae is morphologically also similar to the Brazilian M. warmingii, which differs in its much larger plants and prominent basal labellum lobes.