Grateloupia ramosa H.W.Wang & R.X.Luan sp.nov.(Halymeniaceae,Rhodophyta)is newly described from Hainan province,southern China.It has these morphological features:(1)Thalli were 5-10 cm in height,purplish red in color,cartilaginous and lubricous in texture,Blades had numerous branches and an opposite arrangement.percurrent axes were compressed.Terminal branches consisted of multiple claw-like branches,up to five,which gradually tapered to the upper part of the branches ;(2)The cortex was consist of 3-6 layers of elliptical or anomalous cells,and a medulla consisting of compact medullary filaments that were 2–7 μm in diameter and ran in various directions.The medulla cells approached the inner cortical layer,which consisted of rounded or irregularly shaped cells;(3)Reproductive structures distributed throughout the thallus,especially centralized at the bottom of the end portion of the branches and(6)Carpogonial branch was 4-celled and auxiliary-cell branch was 3-celled,both of them were one of Grateloupia-type.The morphological differences were supported by molecular phylogenetics using ribulose-1,5-bisphosphate carboxylase/oxygenase(rbcL)gene sequence analysis.We compared the rbcL sequences of the six G.ramosa samples to the selected 37 species within Grateloupia and four species in other genera within Halymeniaceae.Partial sequences of 1251 base pairs(bp)corresponding to positions 101 to 1351 of the full rbcL gene(1467 bp)were used for the alignment.There is only 1 bp divergence between the specimens collected from Wenchang and Lingshui of Hainan province.The new species was embedded in the large Grateloupia clade of the Halymeniaceae.The pairwise distances between G.ramosa and other species within Grateloupia ranged from 26 to 105 bp,within the pairwise distances of 13–111 bp between species of the large genus Grateloupia in Halymeniaceae.Thus,we propose this new species as G.ramosa H.W.Wang & R.X.Luan sp.Nov.