The ecoregion covers the waters offshore of the southern Brazilian states of
Rio Grande do Sul and
Santa Catarina. It stretches along of sandy beach coast, and out to sea. It is one of four coastal marine ecoregions in the
Warm Temperate Southwestern Atlantic marine province. It is thus part of the
Temperate South America region. The ecoregion is bounded on the north at
Florianópolis (latitude 28°S), and on the south at the border with
Uruguay (latitude 34°S), where the ecoregion transitions to the
Uruguay - Buenos Aires Shelf marine ecoregion. Together the two ecoregions make up the
South Brazil Shelf, a
large marine ecosystem. The bordering coast is generally low-lying coastal scrub and sand in a long barrier island fronting a chain of lakes and lagoons. The terrestrial ecoregion onshore is the
Uruguayan savanna ecoregion. There are no major rivers feeding directly into the ocean in the ecoregion. A significant feature of the ecoregion is the inclusion of an inland lagoon –
Lagoa dos Patos, the largest coastal lagoon in South America. The lagoon is long and is separated from the ocean by a wide sand bar; it is open to the sea at an opening at the city of
Rio Grande, Rio Grande do Sul. Just south of Lagoa dos Patos is
Lagoon Mirim, which is cut off from the sea. The
continental shelf along this coast of Brazil extends an average of 200 km out to sea before dropping off. The deepest point in the ecoregion overall is , and the average is . The continental shelf is smooth and flat, with the bottom mostly sand and mud. ==Currents and climate==