The genus Neosiphonia Kim and Lee 1999 contains approximately 30 species worldwide. Unidentified samples from Peru are here described as a new species, Neosiphonia ramirezii sp. nov., on the basis of morphological and molecular data. N. ramirezii sp. nov. is characterized by the presence of a limited prostrate system, well-developed erect filaments, rhizoids cut off from pericentral cells by cross walls, four pericentral cells that are completely ecorticate, scarce trichoblasts, inconspicuous scar cells, procarps with three-celled carpogonial branches, spermatangial branches developed from basal cells of forked trichoblasts, and spirally arranged tetrasporangia. Our new species is distinct from N. flaccidissima (Hollenberg) Kim et Lee, N. sphaerocarpa (Børgesen) Kim et Lee, and N. savatieri (Hariot) Kim et Lee from the Pacific temperate coast of South America and from 14 Neosiphonia species reported worldwide by having limited prostrate filaments attached by numerous rhizoids, dichotomous ("Y" shaped) branches in the main axes, and scarce trichoblasts. Phylogenetic rbcL analyses confirm the placement of the new taxon as a distinct species in the genus Neosiphonia. © The Korean Society of Phycology.