Appreciation of the true species diversity of the genus Ulva in Australian waters has been blinkered by the unproved assumption that its representatives there are largely cosmopolitan. As species of Ulva are some of the longest-standing and most widely reported taxa of macroalgae, the presumption that they are worldwide in distribution has led to most Australian members being equated with species originally described from extra-Australian type localities. Ulva species can be notoriously difficult to identify due to the few and often variable characters on which classical taxonomic studies focus so that names of specimens in hand, as well as names appearing in historical distribution records, are frequently difficult or impossible to verify. The combination of morphological and molecular analyses, the latter involving both nuclear (internal transcribed spacer [ITS]) and plastid (rbcL) markers, is critically important in taxonomic studies of the genus and has here been applied to selected Ulva populations from mostly cool-temperate southern Australian localities. It has been determined that habit- and anatomy-based keys of standard taxonomic literature are largely adequate for assigning species names based on classical concepts, but they often obscure a number of cryptic and pseudocryptic species that do not conform to extra-Australian populations of the same designation, as indicated by the corresponding molecular data. Here, we present six species (Ulva australis Aresch., U. compressa Forssk., U. fasciata Delile, U. intestinalis L., U. laetevirens Aresch., U. tanneri H. S. Hayden et J. R. Waaland) for which anatomical and molecular data were congruent with both classical concepts and GenBank accession data and confirm these as cosmopolitan taxa in Australia. We also present six putative species designations based on anatomy [U. clathrata (Roth) C. Agardh, U. flexuosa Wulfen, U. linza L., U. prolifera O. F. Müll., U. stenophylla Setch. et N. L. Gardner, U. brisbanensis sp. nov.] that are inconsistent with molecular data, suggesting novel or cryptic taxa not represented in GenBank.