ABSTRACTHypoglossum rhizophorumsp. nov. is described from deep coral reef habitats on the southern insular shelf of Puerto Rico and the western insular shelf of Basse Terre, Guadeloupe, occurring from 21 to 85 m in depth. The new species is also known from a single shallow‐water collection in Hawaii. Plants grow from a prostrate nonfoliose rhizome producing simple, delicate erect blades at regular intervals. Reproductive plants were not collected in the field; isolated plants, however, readily reproduced in culture demonstrating a typical“Polysiphonia‐type” life history. Spermatangial sori form coalesced oblique bands which are interrupted by secondary cell rows. Tetrasporangia are produced from both lateral and transverse pericentral cells, adjacent second‐ and third‐order cells as well as cor