The genus Polysiphonia Greville is characterized by having four ecorticated pericentral cells, rhizoids in open connection with the pericentral cells, four-celled carpogonial branches, spermatangial branches replacing the whole trichoblast, and tetrasporangia in straight series. Polysiphonia dokdoensis sp. nov. collected from Dokdo, Korea is here described as a new species on the basis of morphological and molecular data. Polysiphonia dokdoensis is characterized by having four ecorticated pericentral cells, unicellular rhizoids in open connection with pericentral cells, scar cells placed between two pericentral cells, an alternating arrangement of unilateral branch pairs, and tetrasporangia in straight series. The alternating arrangement of unilateral branch pairs and the placement of the scar cell between two pericentral cells are recognized as main characters to identify P. dokdoensis. Polysiphonia dokdoensis is morphologically identical to "P. atlantica" sensu Kim and Lee, which is now recognized as P. dokdoensis sp. nov. and distinguished from the authentic European P. atlantica by the distinguishing morphological character states. Phylogenetic analyses of rbcL confirm the placement of the new taxon as a distinct species in Polysiphonia sensu stricto.