To elucidate the taxonomy and phylogeny of Phymatolithon purpureum (P. Crouan & H. Crouan) Woelkerling & L.M. Irvine, we observed the type specimens and fresh samples using SEM (ultra-morphology) and analysed DNA sequences. Phymatolithon purpureum was originally described as Lithothamnion purpureum P. Crouan & H. Crouan from Mingant, Brittany, France. Our molecular phylogenetic analyses using psbA and COI–5P regions placed our collections from Ireland, UK and France in a clade with ‘P. borealis W.H. Adey, J.J. Hernandez-Kantun & P.W. Gabrielson (MH252286)’ from Iceland and ‘uncultured Corallinales (GQ917711 and GQ917512)’ from Roscoff, Brittany, France, near the type locality of P. purpureum. We show that P. purpureum is conspecific with P. borealis and P. polymorphum f. papillatum (Foslie) Foslie based on morphology and molecular data. Also, although P. purpureum has been often confused with P. laevigatum (Foslie) Foslie because of their similar morphology (e.g. smooth surface and sunken tetra/bisporangial conceptacles), our molecular phylogenetic analyses indicate that P. purpureum and P. laevigatum are sister taxa. Our sequences from lectotype material of P. laevigatum and syntype material of Lithothamnion emboloides Heydrich are identical.