Mosla dadoensis (Lamiaceae), a new species from the southern islands of South Korea, is described and illustrated. The new species is morphologically similar to M. chinensis, but is distinguished from the latter by having two types of hairs on its stems, wider leaf blades, longer corolla length, and ellipsoid nutlets with a narrowly U-shaped extended area of abscission scar. Mosla dadoensis is also distinguished from the Chinese narrow endemic M. hangchouensis by having an included pistil to the corolla, smaller ellipsoid nutlets, and later flowering and fruiting season. Phylogenetic analyses, based on two nuclear ribosomal (ETS, ITS) and three chloroplast (rbcL, matK, trnL-F) DNA regions, confirmed that the new species was constructed as monophyletic, and that M. dadoensis and M. hangchouensis form a sister group with robust support. We hereby provide a detailed morphological description of M. dadoensis with its corresponding geographical distributions, and comparison tables of related taxa.