The classification of Cystoclonium obtusangulum has been questioned since the species was first described by Hooker and Harvey as Gracilaria? obtusangula. The objective of this study was to provide the first comprehensive taxonomic analysis of Cystoclonium obtusangulum, based on DNA sequences coupled with morphological observations made on syntype specimens and new collections. Sequence divergences of rbcL, UPA, and COI‐5P, and maximum‐likelihood phylogenies for rbcL and 18S demonstrated that specimens identified as Cystoclonium obtusangulum represent a clade of two distinct species that are distantly related to the generitype Cystoclonium purpureum. A new genus, Meridionella gen. nov., is proposed for this clade. The two species placed in this new genus were morphologically indistinguishable cryptic species, but have disjunct distributions, with Meridionella obtusangula comb. nov. found from temperate to cold coasts of South America and the Falkland Islands and Meridionella antarctica sp. nov., occurring in Antarctic waters. Vegetative and reproductive characters of Meridionella gen. nov. are described, and implications of our results for the biogeography of the family Cystocloniaceae are discussed.