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Rhynchosporamesoatlantica sp. nov. (Cyperaceae) is described, illustrated, and compared with morphologically similar species. Rhynchosporamesoatlantica is known only from southern Delaware, southeastern Maryland, and southern New Jersey, all within the Mid-Atlantic region of the U.S.A. It inhabits sunny, wet margins of natural, shallow, nutrient-poor, seasonal ponds of the Coastal Plain. Narrow leaf blades; fruits obpyriform in outline; faces of mature fruits possessing a central, pale, well-demarcated disk; and fruit tubercle margins denticulate for most of their lengths indicate R.mesoatlantica is most similar to R.filifolia and R.harperi . Rhynchosporamesoatlantica is unique in its fruit dimensions, scales intermediate in length between R.filifolia and R.harperi , and relatively long fruit stipe. The NatureServe rank of Critically Imperiled and the IUCN rank of Endangered appear warranted for R.mesoatlantica because only six populations are known to be extant, most quite small and isolated; all populations occur within a small geographic area; populations have declined; and serious threats confront the survival of the species., Competing Interests: The authors have declared that no competing interests exist., (Amanda Treher Eberly, Robert F. C. Naczi.)