Galeoglossum cactorum , a new species of Orchidaceae from the Mixteca Alta region in the state of Oaxaca, Mexico, is described and illustrated. The new species is florally similar to G. thysanochilum but differs from it in the linear-oblanceolate leaves subtended by imbricating, dark brown cataphylls, presence of a papillose thickening near the base of the labellum, proportionately shorter, obtrapezoid column with lateral laminar wings and complete fusion of the filament, and prominent apicule of the rostellum remnant. A cladistic parsimony analysis of DNA sequences of nuclear ITS region and plastid matK-trnK region showed that the three currently recognized species of Galeoglossum form a monophyletic group, which is sister to Prescottia. Galeoglossum cactorum is known from a single locality in an unusual habitat for orchids, i.e. a xerophilous scrub dominated by columnar cacti of the genus Neobuxbaumia, whereas both G. thysanochilum and G. tubulosum dwell in oak- and pine-oak forest.