The taxonomy of the marine species of Collemopsidium in northwest Europe was investigated using morphological and molecular evidence. 210 specimens were collected from the west coasts of Norway and Ireland, and morphological and ecological variables recorded. ITS1 rDNA sequences were obtained from 24 specimens. A phylogenetic analysis, resulting in a single optimal tree, was performed under the unweighted least squares optimality criterion based on maximum likelihood distances obtained from unaligned sequences. Principal components analysis (PCA) was performed on morphological variables of the sequenced specimens, and classification was carried out by maximizing agreement between the phylogenetic tree and the PCA. Thallus immersion, and perithecial immersion and size, were the most important characters for discriminating between taxa. Apart from substratum, niche separation between taxa was small but statistically significant as shown by a redundancy analysis (RDA). Variance partitioning indicated that genetic variation is vastly more important than ecology for explaining phenotypic variation. Five species of marine Collemopsidium are recognized, including two new combinations: C. foveolatum (syn. Arthopyrenia foveolata) and C. ostrearum (syn. Lecanactis ostrearum) combs. nov.