Arthrostemma primaevum, a new species endemic to southeastern Mexico, is described, illustrated, and compared with its closest extant relative,A. ciliatum. The chromosome count ofn=15, reported here forA. primaevum, suggests thatA. ciliatum, withn=30, is a tetraploid derivative with a much broader geographic and elevational range. In addition to its distinctive unlobed staminal appendages and unique chromosome number,A. primaevum is notable for its shorter, urceolate hypanthium and seeds that have essentially smooth continuous semicircular ridges.