Approximately 29 species of Neosiphonia are currently recognized worldwide. We describe our unidentified samples collected from the Pacific coast of South America as a new species, N. peruviensis sp. nov., based on morphological and molecular data. Neosiphonia peruviensis sp. nov. is characterized by erect and prostrate thalli, rhizoids cut off from proximal ends of pericentral cells by cross walls, six peri-central cells that are totally ecorticate, abundant trichoblasts forked once or twice, prominent scar cells and tetrasporan-gia spirally arranged. Our new species is similar to several Neosiphonia/ Polysiphonia species having more than four pericentral cells: N. notoensis, N. porrecta, "N. tepida", N. teradomariensis, and "P. forfex". However, N.peruviensis sp. nov. is distinguished from these similar species by having six pericentral cells through all of the thallus. Phylogenetic analyses of rbcL indicate that our N. peruviensis is placed in genus Neosiphonia and is also distinguishable from other Neosiphonia species. © 2012 by Walter de Gruyter Berlin Boston.