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SUMMARY: The red algal genus Dudresnaya (Dumontiaceae, Gigartinales) has traditionally been a morphologically well‐defined taxon, but its molecular phylogeny has rarely been studied. To examine the phylogenetic relationships among Dudresnaya species, we generated new partial sequences of mitochondrial cox1, chloroplast rbcL and nuclear 28S rRNA genes from an undescribed Dudresnaya species from Okinawa Island, Japan, alongside five additional described species. Our phylogenetic analyses show that Dudresnaya is genetically diverse and polyphyletic. Based on molecular phylogeny and morphological data, we describe the Okinawan Dudresnaya as a new species, Dudresnaya ryukyuensis, and transferred Dudresnaya minima and Dudresnaya littleri, which were phylogenetically and morphologically distinct from the genuine Dudresnaya, to the new genera Himehibirhodia and Nudresdaya, respectively. Our phylogenetic analyses also showed that the Dumontiaceae is not a monophyletic group including the Gainiaceae and Rhizophyllidaceae (DGR complex). Considering that the DGR complex exhibits female reproductive structures and their post‐fertilization development that are similar to each other, the DGR complex appears to be recognized as the Dumontiaceae sensu lato. [ABSTRACT FROM AUTHOR]