Five new species in the long-jawed orb-weaving spider genus Tetragnatha Latreille, 1804 are described from South America: Tetragnatha amazonicasp. nov. (Venezuela); T. cristatasp. nov. (Argentina and Brazil); T. didoratasp. nov. (Brazil); T. oncognathasp. nov. (Brazil); and T. pradoisp. nov. (Argentina and Brazil). A key to the 21 species of Tetragnatha from Argentina and Brazil is provided, completing the revision of the genus for these countries. The female of T. cladognatha Bertkau, 1880 is redescribed and a neotype is proposed, and its male is described for the first time. Additionally, we update the taxonomic status of the following six South American species: Tetragnatha labialis Nicolet, 1849 and T. americana Simon, 1896 are considered new junior synonymies of T. nitens (Audouin, 1826); and Tetragnatha bishopi Caporiacco, 1947, T. linearis Nicolet, 1849, T. similis Nicolet, 1849 and T. sternalis Nicolet, 1849 are proposed as nomina dubia due to missing or juvenile type-material.