There has been much taxonomic and nomenclatural confusion with the previously reported flattened species of Graci/aria from the western Atlantic. Most records are poorly known, inadequately characterized, or misidentified. Based on phylogenetic analysis of chloroplast-encoded rbcl... sequences and comparative morphological studies of flattened species of western Atlantic Graci/aria, two major lineages are identified: the" Graci/aria venezue/ensis clade," comprising G. venezue/ensis W. Taylor, G. intermedia J. Agardh subsp. intermedia, G. intermedia subsp. ganesana subsp. nov., and G. yoneshigueana sp. nov.; and the" Graci/aria mammillaris clade," comprising G. mammil/aris (Montagne} Howe, G. galetensis sp. nov., G. hayi sp. nov., G. o/iveirarum sp. nov., and G. smithsoniensis sp. nov. Of the flattened taxa included in these two clades, 5 new species and 1 subspecies are described, and the other flattened species are clarified and characterized. Our findings show that the number of compressed to flat Graci/aria taxa in the western Atlantic has been underestimated and includes previously overlooked or inadequately described taxa with validly published names.