Didymodon baii D.P. Zhao, J.N. Wang & X.D. Zhao (Pottiaceae) is described and illustrated as a new moss species from Inner Mongolia Autonomous Region of China. It differs from congenerics by its oblong-lanceolate leaves gradually narrowed to the apex, with plane margins throughout, and costa long-excurrent as a flexuose subula, the transverse section of the costa having 4–5 guide cells in one layer and ventral stereids absent to weakly differentiated in one layer. Its relationships with some closely similar Didymodon species are discussed.