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Haraldiophyllum hawaiiense sp. nov. is described as a new mesophotic alga and a new genus record for the Hawaiian Islands. Six specimens were collected at a depth range of 81-93 m from Papahānaumokuākea Marine National Monument, and their morphology investigated, as well as molecular phylogenetic analyses of the plastidial ribulose-1,5- bisphosphate carboxylase–oxygenase large-subunit (rbcL) gene and a concatenated alignment of rbcL and nuclear large-subunit rRNA gene (LSU) sequences. Phylogenetic analyses supported H. hawaiiense sp. nov. as a distinct lineage within the genus Haraldiophyllum, and sister to a large clade containing the type species, H. bonnemaisonii, as well as H. crispatum and an undescribed European specimen. The six Hawaiian specimens were shown to be identical, but unique among other species of the genus as well as the recently segregated genus Neoharaldiophyllum, which comprises half of the species previously included in Haraldiophyllum. The vegetative morphology of H. hawaiiense sp. nov. resembles Neoharaldiophyllum udoense (formerly H. udoensis); however, no female or post-fertilization structures were found in the Hawaiian specimens to allow a more comprehensive comparison. The molecular phylogenies demonstrate that Haraldiophyllum is paraphyletic, suggesting either that the Myriogrammeae tribe includes undescribed genera, including Haraldiophyllum sensu stricto, or that Neoharaldiophyllum species should be transferred into the genus Haraldiophyllum. However, based on vegetative morphology and molecular analyses, and pending resolution of this taxonomic issue, the Hawaiian specimens are placed within the genus Haraldiophyllum. This new record for the Hawaiian Islands highlights the novel biodiversity from mesophotic depths, reaffirming the need for further investigation into the biodiversity of Mesophotic Coral Ecosystems.