Phymatolithon atlanticum is newly described from France and Northern Ireland (the United Kingdom) using both molecular (rbcL sequence data, concatenated psbA and COI-5P datasets) and morpho-anatomical analyses. The species shows sequence divergences from other Phymatolithon species by 10.7–15.3% in rbcL, 5.1–9.2% in psbA and 9.6–16.8% in COI-5P. Morpho-anatomically, P. atlanticum is encrusting to lumpy, has slightly sunken female conceptacles with raised rims, lacks buried conceptacles, has five to six rosette cells surrounding tetra/bisporangial conceptacle pores, and has tetra/bisporangial conceptacle roofs that are pitted with depressions as a result of the disintegration of rosette cells. This latter feature is what currently separates the species morpho-anatomically from all other species of Phymatolithon. The tetra/bisporangial conceptacle of P. atlanticum closely resembles those of Lithothamnion crispatum and Leptophytum bornetii and may well represent additional examples of convergent evolution in the non-geniculate coralline algae.