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Historically, Porphyra purpurea (Roth) C.Agardh was the only monostromatic species recorded from the Northwest Atlantic that had a sectored male/female blade. Recently, two additional monostromatic taxa with sectored blades have been reported from this region. One is the newly described species P. birdiae Neefus et Mathieson and the other is P. katadae A.Miura, an introduced Asian species. In this study, a combination of molecular, morphological, and ecological evaluations have been used to characterize the identities, distributions, and relationships of these three taxa with sectored blades. Interspecific divergence in rbcL sequences ranged from 7.2% to 9.6%, while intraspecific variability in rbcL, ITS1, and SSU was very low (0-2 bp). While P. purpurea has a broad distribution from northern Nova Scotia (Canada) to western Long Island Sound (New York, USA), P. birdiae occurs only north of Mount Desert Island (Maine, USA) and P. katadae is restricted to an area south of Cape Cod (Massachusetts, USA). The three species can be further distinguished by combinations of blade thickness, colour, intertidal position, substratum, and seasonality. Molecular variability in Northwest Atlantic Porphyra purpurea populations, showed that several specimens were found to have rbcL sequences 100% identical to a GenBank accession for P. rediviva Stiller et Waaland, while other specimens were 100% identical to the neotype of P. purpurea with 1 bp substitution between the two species. Comparison of the rbcL and ITS I sequences of the holotype and isotypes of P. rediviva were 100% identical to the neotype and isoneotype of P. purpurea. Based upon these molecular findings and the failure of morphological and ecological features to clearly delineate the two taxa, P. rediviva is synonymized with P. purpurea.