Abstract Grateloupia huertana sp. nov. is newly described from Oaxaca, Pacific Mexico on the basis of comparative morphology and rbcL sequence analysis. It is distinguished from other Grateloupia species by its entirely prostrate, fleshy-cartilaginous, highly polymorphic habit ranging from single, irregularly spreading and perforated lobes with marginal proliferations, to rosette-like clumps of overlapping blades up to 5 cm in diameter. The entire lowermost surface acts as a single expanded holdfast conforming to the topography of the substratum. Globose and deeply immersed cystocarps are scattered and restricted to the dorsal side. Auxiliary cell and carpogonial ampullae are of the simple Grateloupia-type. The medulla varies from compact and cellular to a network of predominantly periclinal filaments producing varying degrees of secondary cells as the thallus expands laterally. The new species is embedded within the Grateloupia clade in rbcL trees, but its phylogenetic position in relation to other membe...