Due to the high plasticity of coralline algae, identification based on morphology alone can be extremely difficult, so studies increasingly use a combination of morphology and genetics in species delimitation. A DNA barcoding study was carried out on maerl-forming coralline algae using the mitochondrial cytochrome oxidase 1 gene, CO1, and the plastid gene, psbA, on field specimens from Falmouth and Oban together with herbarium specimens from the Natural History Museum, UK, and the Smithsonian Institution, Washington, USA. Results revealed the presence in the north of Britain of a new species, Lithothamnion erinaceum Melbourne & J. Brodie, sp. nov., which was previously misidentified as Lithothamnion glaciale. The results also indicated that Lithothamnion lemoineae, which had earlier been recorded from Britain, was not present. One of the biggest concerns at present is how organisms will respond to climate change and ocean acidification, and it is imperative that investigations are put on a firm taxonomic basis. Our study has highlighted the importance of using molecular techniques to aid in the elucidation of cryptic diversity.