Halimeda J.V.Lamour. is a green algal genus that is an important component of tropical reefs and lagoons. Although it does not generally occur outside the tropics, the range of one putatively widespread species, Halimeda cuneata Hering, in Western Australia extends southward past Cape Leeuwin to its southern extreme at Cape Howe and around to its most easterly record at Middle Island of the Recherche Archipelago. Previous molecular studies have shown that H. cuneata as recorded worldwide encompasses cryptic species diversity, with most of the cryptic entities being geographically isolated from the others. Halimeda cuneata has been the name consistently applied to specimens from the south-western coast of Australia, where it has been regarded as the only representative of its genus in the region. Using a combination of morphological features and assessment of species boundaries based on a plastid gene (tufA) and a nuclear protein-coding gene (HSP90), we have reassessed the supposed presence of H. cuneata in south-western Australia. Our results showed the existence of two species in the region, namely, H. versatilis J.Agardh, to which the name H. cuneata has been misapplied, and H. gigas W.R.Taylor, a central-Pacific species previously recorded only from tropical Australia.