Lampisiphonia iberica gen. et sp. nov. is described on the basis of specimens collected from subtidal rocky bottom habitats on the Atlantic coast of the Iberian Peninsula. The new genus was distinguished by an erect habit, pseudodichotomous branching, and 9-11 pericentral cells. Plants were bright red to brown-red in colour, 3-6 cm high, firm at the base but ultimate divisions soft and flaccid. Cortication was thick but restricted to the base of large thalli. Plants were attached to the substratum by discoid holdfasts with the tips of decumbent branches reattaching by secondary discoid holdfasts or rhizoids cut off from pericentral cells. Trichoblasts and scar cells were absent and the branching was exogenous. Tetrasporangia were arranged in straight series of up to 15 segments. Sexual structures were not observed. Lampisiphonia was separated from other groups and genera of Polysiphonia sensu lato and the tribe Polysiphonieae by a combination of features that include the absence of trichoblasts and presence of a compact basal cortication. Furthermore, Lampisiphonia had tetrasporangia in a straight series, a feature to date considered unique to Polysiphonia sensu stricto. The phylogenetic relationship of Lampisiphonia among the three resolved lineages of the Polysiphonia sensu lato was equivocal in our analyses and it was not clearly distinct from the Neosiphonia group and the multipericental group in morphological and molecular characters. Nevertheless, monophyly of Lampisiphonia and the Polysiphonia group was statistically rejected in small-subunit ribosomal DNA, rbcL and combined data with the Shimodaira-Hasegawa test. We therefore proposed Lampisiphonia as a new genus.