Ulva armoricana sp. nov. is described from the coast of Brittany, France, using the taxonomic criteria established by Bliding, Koeman and van den Hoek for the European species of Ulva. Morphological evidence strongly suggests that Ulva armoricana is conspecific with the free-living Ulva populations responsible for the ‘green tide’ phenomenon that occurs at some eutrophicated sites in North Brittany. Blades of U. armoricana are variable in shape, fragile, easily torn, and 30–55 μm thick in middle and apical regions; they differ from those of U. olivascens by their pale green colour and from those of Ulva lactuca, U. rotundata, U. gigantea, U. pseudocurvata and U. curvata by their crumpled texture that creates an irregular light-reflecting surface, and by the presence of tooth-like protuberances at the margin of the blade. U. armoricana differs from U. scandinavica in having longitudinal ribs containing closely packed bundles of rhizoids, except in the youngest thalli. U. armoricana can be discriminated from U. rigida by the polygonal (never rounded) cells in the mid-thallus region, and quadrangular cells arranged in small longitudinal rows in rapidly growing parts. Pyrenoid number, 1–2 (3–4), overlaps with that of other species, and shape of non-rhizoidal cells of the basal region in transverse sections, either oval or with tapered ends, appears not to be a good taxonomic character. Herbarium data indicate that the species has been collected in northern France since at least 1869, but was previously unrecognized.