A new species, Stillingia yungasensis, from the “Yungas Tucumano-Bolivianas” forest (northwestern Argentina and southern Bolivia) is described and illustrated, including data on habitat, distribution, phenology, and a key to the new species and closest taxa. Stillingia yungasensis may be one of the most basal taxa within Series Oppositifoliae. No other species of Stillingia is sympatric with S. yungasensis, and its distribution supports a recent subdivision of the Yungas forests into two distinct phytogeographical units. Unlike the remaining species of Series Oppositifoliae, S. yungasensis is relatively frequent within its habitat.