A new highland species of chestnut-capped Arremon brush-finch is described from the Sierra Madre del Sur of central Guerrero. This form, although indistinguishable in external phenotype from adjacent populations to the east in Oaxaca, is dramatically differentiated in mitochondrial DNA sequence characters, and quite unexpectedly is the sister lineage to the very distinct (phenotype and genotype), central-Mexican-endemic A. virenticeps. Nuclear sequence differentiation in the new lineage is more subtle than in mitochondrial DNA, but is on par with that in the well-marked A. virenticeps. The new species is thus distinct from its sister lineage in genotype and phenotype, and clearly distinct from all other forms in genotype; however, it has retained an ancestral external phenotype similar to other members of the broader A. brunneinucha complex.