Monardella angustifolia Elvin, Ertter & Mansfield (Lamiaceae), a new species from southeastern Oregon and adjacent Idaho, is described and illustrated. It occurs on ash tuff outcrops in and near Leslie Gulch in the northern reaches of the Great Basin Desert. The new species is best distinguished by its calyx trichomes, fascicled leaves, and leaf and bract morphology. It is unique in the genus in having very narrow, conduplicate, and often fascicled leaves and being endemic to soils derived from ash tuff. It is most similar morphologically and ecologically to several perennial species of Monardella Benth. in the Mojave Desert ca. 900 km to the south. It shares with them a similar habit, pubescence, branching, and bract and glomerule morphologies.