Didymodon oedocostatus J. A. Jiménez & M. J. Cano is described and illustrated as a new species from Catamarca and Tucumán provinces in northwestern Argentina. The new taxon is characterized mainly by its appressed and imbricate leaves when dry, the red to orange reddish color in KOH, abundant propagulae in the axils of the leaves, smooth laminal cells, a wider costa at midleaf than below, and the presence of a bulging ventral costal pad of cells. Its distinction from and relationships with some closely related species within the genus are discussed.