Osmundea purepecha Sentíes, Mendoza-González & Mateo-Cid, n. sp., is proposed from Zapote, Michoacán, tropical Mexican Pacific coast, on the basis of comparative morphology and rbcL sequence analysis. This species has characteristics typical of the genus Osmundea, including two pericentral cells per axial segment and male gametophytes with a “filament” type reproductive structure. Spermatangial branches are simple, terminating in one or two spherical sterile cells, developing in pocket-shaped pits located at tips of branchlets. Phylogenetic position of this species was inferred by analysis of chloroplast-encoded rbcL gene sequences from 33 taxa, including one member of the Ceramiaceae as an outgroup. The Mexican Pacific sample, from Zapote, formed a distinctive and well-supported basal clade segregated from other Osmundea species. Osmundea purepecha, n. sp., is morphologically and genetically distinct from other Osmundea species, supporting the assignment of this new taxon.