Phantomia vadimii S. He & W. R. Buck, a new moss genus and species, is described from Lào Cai Province, northern Vietnam. The new taxon is characterized by its abruptly convolute leaf shoulders; the presence of a median row of stereid cells in the upper lateral costal cross sections; a weakly differentiated marginal border at the leaf base; and the absence of a central strand in the stem. The new genus shares some characteristics with Chorisodontium (Mitt.) Broth. in having a narrow basal costa that flares outward at the midleaf and persistent, inflated alar cells. It also has an anatomical similarity to the leaf costal cross sections (especially the upper leaf costa) of Dicranodontium Bruch & Schimp. The similarities and differences of the new genus in comparison with the genera of Campylopodioideae and Dicranoideae are discussed.