Polysiphonia sensu stricto is characterized by having 4 ecorticate pericentral cells, rhizoids in open connection with the pericentral cells, four-celled carpogonial branches, spermatangial branches replacing the whole trichoblast, and tetrasporangia arranged in straight series. Polysiphonia ulleungensis sp. nov. is newly described from Sadongri, Ulleung Island, Korea, based on morphological and molecular evidence. It is mainly characterized by having ecorticate axes with 4 pericentral cells, apical cells transversely or obliquely divided, unicellular rhizoids in open connection with pericentral cells, very scarce trichoblasts and scar cells, procarps with a four-celled carpogonial branch, and spermatangial branches replacing the whole trichoblast. Polysiphonia ulleungensis is closely similar in morphology to P. atlantica sensu lato. We concluded that materials of P. atlantica sensu Nam and Kang from Korea correspond to P. ulleungensis. By contrast, the new species differs morphologically from the Atlantic specimens of P. atlantica as well as from P. atlantica sensu Kim and Lee from Korea. Morphological characteristics and rbcL sequence analyses support the taxonomic placement of P. ulleungensis within Polysiphonia sensu stricto.