Abstract.The existence of cryptic diversity in the Porphyra leucosticta complex of the North Atlantic and Mediterranean was confirmed from a wide variety of recent and over 100-year-old herbarium specimens using plastid and nuclear molecular data in combination with karyological, morphological and ecological observations. The results revealed the presence of a new species, P. olivii sp. nov., within the P. leucosticta complex. Possible evidence for sexual reproduction was observed in a culture of P. olivii from the holotype site, Gulf of Thessaloniki in Greece, where n=2 chromosomes in male gametes and neutral spores, and 2n=4 in zygotospores. Archeospores and neutral spores also formed in culture providing evidence for possible asexual reproduction. The results also revealed the presence of P. rosengurttii, originally described from the north-western Atlantic, in both the north-eastern Atlantic and the Mediterranean, and enabled new morphological and ecological information to be obtained for this species. The rbcL sequences for P. olivii, including from an 1889 specimen of Porphyra leucosticta, differed by one transition between the north-western Atlantic and Mediterranean. The ITS1 of the holotype of P. olivii differed by a single bp transition from another specimen from the same location. The ITS1 of specimens from the northwestern Atlantic were identical but differed by 3-4 bp substitutions from the specimens from the Mediterranean. Partial rDNA SSU sequences were identical. In P. rosengurttii, rbcL sequences, including from specimens dated 1890 and 1906, differed by a single transition between both regions, the ITS1 had 92100% similarity and partial rDNA SSU sequences were identical. Our study indicates that molecular, morphological and ecological evidence from a wide variety of samples (recent and old) and locations should be investigated before a new species is erected. The molecular comparison with old herbarium specimens also revealed that P. olivii and P. rosengurttii are not recent invaders to the Mediterranean but have been present for at least 100 years.