The marine red alga Lomentaria benahoarensis sp. nov. from the Canary Islands is described. Plants grow epiphytically on Corallina elongata forming turfs of creeping axes attached by lateral discoid holdfasts and erect axes up to 12 mm high. Erect axes are terete to slightly compressed, 200–300(–1000) μm broad, radially branched up to three orders, with laterals progressively thinner, constricted only at the bases of terminal axes. The cortex is three-layered with the outermost cortical cells arranged in a near-continuous surface layer. Medullary filaments form a loose network of narrow elongate cells, which bear secretory cells. Gametophytes are dioecious with spermatangia formed in continuous spermatangial sori; cystocarps are urceolate and ostiolate. Tetrasporangia are subspherical and formed in depressed rounded sori in slightly swollen regions in the middle of fertile terminal axes. The new species has no single unique feature, but differs from the other Lomentaria species by a distinctive combination of attributes. Closest related species to Lomentaria benahoarensis appear to be L. corallicola , L. gracillima , L. monochlamydea , L. orcadensis , L. rawitscheri , and L. tenerrima . Although all these species have a diminutive, decumbent habit and lack regular constrictions in axes, the number of cortical cell layers and the arrangement of outer cortical cells distinguish them.