The dinoflagellate cyst (dinocyst) Diconodinium lurense sp. nov., was recovered from wells, boreholes and outcrops from several Southwestern Atlantic basins (figure 1). It is present in the Maastrichtian-Danian Pedro Luro Formation (Kaasschieter, 1963), Colorado Basin, in an onshore borehole and from offshore wells. The species was also recorded as Diconodinium sp. in coeval deposits of the Maastrichtian-Mid Eocene Gaviotín Formation in an offshore well in the Punta del Este Basin, Uruguay (Daners and Guerstein, 2004). In both basins, the distribution of Diconodinium lurense seems to be consistent and characterizes the Cretaceous-Palaeogene (K/P) boundary. And, well preserved specimens of this species from surface section of the Calafate Formation in the Austral Basin, allow determination of its first occurrence datum (FOD) in the latest Maastrichtian (Marenssi et al ., 2004). The aim of the present contribution is to formally describe the new species and to emphasize its regional biostratigraphical potential in the Southwest Atlantic basins.