The new moss species Polytrichadelphus spinosus A.M. Aponte-R. & Uribe is described and its diagnostic features are illustrated. It is recognized by the small teeth on the abaxial leaf surface, particularly on the costa, from the apex to the sheath, the pyriform to round apical cells of the lamellae, the broad shoulders and the stomata distributed almost over the entire capsule. It occurs in western Colombia, in the departments of Chocó, Risaralda, Huila, Putumayo, Valle del Cauca and Nariño, predominantly in montane forests slopes in open to disturbed forests. A table contrasting the new and sympatric species of Polytrichadelphus is presented.