| Summary: | The genera <i>Monostroma</i> and <i>Gayralia</i> belong to the order of monostromatic green algae; however, their taxonomic delimitation remains controversial at the genus level. This study attempts to address this issue through the combined analysis of the morphology and nuclear-encoded Internal Transcribed Spacer region sequences of monostromatic green algal samples collected in the South China Sea. Our phylogenetic data revealed that the monostromatic specimens were separated into the <i>M. nitidum</i> clade, <i>G. brasiliensis</i> clade, and a single <i>Monostroma</i> sp. clade, and that the inter-genera genetic distance between the <i>Monostroma</i> and <i>Gayralia</i> genera was lower than that observed within the <i>Monostroma</i> genus. All the specimens presented similar morphology in their single cell-layered thallus, with irregularly arranged cells, rounded cell corners, a parietal chloroplast, and predominantly one (>90%) pyrenoid. Their most obvious morphological difference was in thallus thickness and size. Moreover, the monostromatic specimens of the <i>M. nitidum</i> clade corresponded to the morphological description of the <i>M. nitidum</i>-type specimens. The genus <i>Monostroma</i> was erected earlier than the genus <i>Gayralia</i>. Therefore, we propose to assign the genus <i>Gayralia</i> to <i>Monostroma</i> based on the morphological and phylogenetic analysis and genetic distance data presented here.
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