Eight primarily saxicolous Usnea subgenus Usnea species are reported from Argentina and Uruguay: U. amblyoclada (Müll. Arg.) Zahlbr. (syn. nov. U. pinnata Räsänen), U. densirostra Motyka, U. durietzii Motyka (syn. nov. U. caespitia Motyka), U. exigua J.M. Rodr. & P. Clerc sp. nov., U. fastuosa (Müll. Arg.) Zahlbr., U. hieronymii Kremp. (syn. nov. U. hieronymii var. adusta Kremp.), U. lutii J.M. Rodr. & P. Clerc sp. nov. and Usnea saxidilatata J.M. Rodr. & P. Clerc sp. nov. are described with morphological, anatomical and chemical data. Their taxonomy and distribution are discussed. A key to the saxicolous species, including those occurring secondarily on rocks (i.e., U. columbiana Motyka, U. cornuta Körb., U. dasaea Stirt. and U. cf. perhispidella J. Steiner) is provided.