A new rheophytic moss genus and species Yunnanobryon rhyacophilum occurring in fast-flowing rivers of western Yunnan and southern Guizhou Province is described and illustrated. It is a slender moss with stoloniform primary stems and intricately much branched secondary stems, flagellate-attenuate at the tips; small, ovate to ovate-lanceolate leaves with almost uniform areolation throughout the lamina of small, rounded, hexagonal to quadrate-hexagonal cells; coarsely serrate leaf margins in the distal portion; subpercurrent single costae; and very short, bicellular axillary hairs consisting of a large globular apical cell and a short, quadrate and brownish-tinged basal cell. The peculiar shape of the axillary hairs suggests a close alliance of this new genus with the Regmatodontaceae.