Clastobryopsis imbricata sp. nov. (Pylaisiadelphaceae, Musci) is described on the basis of specimens collected at middle elevations of Doi (Mt.) Inthanon, northern Thailand. The species is characterized by strongly imbricate and spiral foliation of short ascending branches, caudate branch tips bearing smooth, filamentous gemmae in leaf axils, carinate leaves with distinct spine(s) at the base of the dorsal side of the keel, thin-walled, quadrate alar cells arranged in a scalariform manner, narrowly cylindrical capsules with perforate exostome teeth and reduced endostome, and non-collenchymatous exothecial cells. The multistratose alar cells, smooth filamentous gemmae, and non-collenchymatous exothecial cells, as well as phylogenetic inferences based on rbcL sequences, suggest that the species belongs in the genus Clastobryopsis (Pylaisiadelphaceae).