Thalassiosira laevis, a rare nanoplanktonic diatom species described from Xiamen Harbor (East China Sea), is for the first time recorded in the South Atlantic Ocean from fossil and modern samples. It was an important fossil diatom indicating sea level changes in the assemblage recovered from a Late Holocene sequence from the Brazilian coast (coastal plain of São Paulo State) reaching an abundance of 53% at about 5000 yr BP. In modern samples, it was a rare species found in plankton and exposed sand sediments from dissipative marine sandy beaches of Rio Grande do Sul State (Praia Azul, Tramandaí and Cassino) and a bay beach in Santa Catarina State (Zimbros). Distribution data shows Thalassiosira laevis as a widespread diatom living along coastal littorals and inlets close to estuarine area mouths.