[in Spanish with English abstract: ] Based on a study of the diversity of the genus Usnea in southern Chile and the review of specimens collected in Patagonia of Argentina, a new species is reported from both countries: Usnea dimorpha (Mül. Arg.) Motyka. Its morphological, anatomical and chemical characteristics are described. Also, its distinctive characters are discussed in relation to other species of the genus in the region. Key words : Ascomycota; forest; lichen; Patagonia; taxonomy.