Colpomenia , a small genus with 11 species of globular to bullate form, occurs in temperate to tropical waters worldwide. Because morphology is highly diverse, the species-level taxonomy requires re-evaluation. We analyzed the mitochondrial cox 3 gene from 50 samples of the genus. A new species, Colpomenia claytonii sp. nov., is described based on samples collected from Korea, Japan, Hong Kong, Australia, New Zealand, South Africa, and the USA and compared with similar congeners. Compared to others, the new species is larger, and has a more irregular thallus often with a deeply infolded surface. It is usually epilithic in tide pools and in the lower intertidal and subtidal zones. In all phylogenetic analyses of cox 3 sequences, C. claytonii was consistently distinct from congeners. Colpomenia expansa is closely related to C. claytonii , and the clade containing these two species is closely related to C. peregrina . A total of 14 cox 3 haplotypes was found in C. claytonii , indicating high haplotype diversity and a recent dispersal pattern. The present study shows that C. claytonii is a widely distributed species across the Pacific Ocean to South Africa; it was previously misidentified as a variant of C. peregrina .