A new species of Trichocline from southern Brazil and Uruguay is described as T. cisplatina. The new species is similar to the allopatric species T. catharinensis in involucre features and phyllary shape. Trichocline cisplatina can be distinguished by its procumbent scape, pinatifid leaves with up to ten pairs of rounded and flexuose lobes that occasionally form secondary lobes, ray florets with a yellow-orange corolla, wider involucre and phyllaries, and an ovary with whitish 2-seriate trichomes that are inflated at the apex and densely distributed. The new species occurs in the southeastern region of Rio Grande do Sul State, Brazil and southeastern Uruguay, on grasslands with sandy or rocky soils or dunes, and is endemic to the region. Here we provide a description of T. cisplatina, information about its conservation status and ecology, a distribution map, illustrations, and a key for identification of the native species of Trichocline from southern Brazil and Uruguay.