Abstract: To explore the species diversity of the marine diatom genus Thalassiosira Cleve, several monoclonal strains were established from Daya Bay, north coast of the South China Sea. The morphology of the cells was examined by light and scanning electron microscopy. The hypervariable D1-D3 region of the nuclear large-subunit ribosomal DNA and the relatively conserved region of the nuclear small-subunit ribosomal DNA were sequenced and used for phylogenetic relationship analysis. Combined the morphology and molecular data, a new species, T. flabellata sp. nov. X H Guo, Y Q Guo & Y Li was described. The cells normally form short chains or solitary. A central fultoportula, one ring of marginal fultoportulae and one marginal rimoportula are present on the valve face. This species is characterized by fasciculate areolae on the valve.