Didymodon buckii J.A.Jiménez & M.J.Cano is described as a new species from central Peru and northwestern Argentina. It is readily distinguished from all congeners by having bistratose to tetrastratose leaf margins from the apex to near the base and long-cylindric, thickened and deciduous leaf apices that are strongly channeled ventrally. Additionally, the new moss is characterized by its linear-oblong leaves, papillose distal laminal cells, and costa with a hyaline area of rectangular cells on the distal ventral surface and with ventral stereids below midleaf. Drawings and light microscope photographs of the main characters are given, and possible confusion with other closely related taxa is discussed. Distribution of the new species is mapped.