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Seaweed latitudinal patterns of diversity present singularities, with implications for ecological, biogeographic and conservation studies. Despite observed global trends, distinct gradients of marine biodiversity occur along the Southwestern Atlantic (SWA) coast. However, the transitional warm-temperate marine province is a major knowledge gap, limiting conclusions about regional seaweed biodiversity. Our study investigated the composition of macroalgal assemblages in rocky intertidal shores between southern Brazil and Uruguay (from 26 degrees S to 35 degrees S), accounting for the regional oceanography and coastline features. Integrating historical and new data, we found a significant latitudinal gradient in seaweed species richness and turnover. The complex thermo-haline structure of the coastal waters, summed to large local fluvial discharges (i.e., La Plata River and Patos-Mirim Lagoon), significantly impacted the distributional pattern of macroalgal assemblages. Sea surface temperature and salinity are the main drivers of the species composition and distribution, despite the changes in coastal geomorphology and substrate availability. Whilst the northernmost section (26 degrees S-29 degrees S) was characterized by a tropical/warm-temperate phycoflora, the southernmost (32 degrees S-35 degrees S) was dominated by eurythermic and euryhaline species. Taking into account this and previous phycogeographic studies, the lowest seaweed species richness seen in SWA rocky shores occurs across 32 degrees S and 37 degrees S, while increasing towards both north (into the tropical marine province) and south (into the cold-temperate marine province). Our study fulfills knowledge gaps in Southwestern Atlantic Ocean biogeography, providing baseline information for conservation efforts and for the detection of ongoing human impacts on intertidal macroalgal assemblages.