Species delimitation in Rhododendron subsect. Caroliniana (Ericaceae) has been disputed in the past with one or three species accepted. Here we report a fourth species, R. smokianum, a narrow endemic from the Great Smoky Mountains (Tennessee/North Carolina, USA). It is characterized by short-tubed corollas and small, compact growth. We support our conclusion by phylogenetic analyses of ITS and plastid trnL-F sequence data. The new species is a specialist of open mafic rock slopes and maintains morphological and phenological differences to the related species even in cultivation.