While undertaking a botanical survey of the Andersen Air Force Base on Guam (Mariana Islands) in 1994, botanists from the National Tropical Botanical Garden collected an unusual suffrutescent, non-aromatic member of the Lamiaceae family growing on limestone cliffs in the northeastern part of the island. Based on morphology and molecular data ( trnLF, matK), it was determined to belong to the genus Pogostemon Desf., a genus previously unknown from the Micronesian, Melanesian, and Polynesian region. Moreover, the analysis also showed that it was not conspecific with P. cablin (patchouli), and of the species available to include in the phylogenetic analyses it is sister to P. hirsutus ¸ a species from India and Sri Lanka. Differing from its congeners by its large, loose inflorescence 2.5–5 cm wide and up to 7 cm wide in fruit, it is here illustrated and described as a new species, Pogostemon guamensis Lorence & W.L. Wagner and its habitat and conservation status are discussed.