The new species, Rhynchospora belizensis, is described, illustrated, and compared with phylogenetically closely related and morphologically similar species. Rhynchospora belizensis is known only from southern Belize, where it inhabits sunny, very wet, nutrient-poor, peaty ground of graminoid-dominated, gently sloping hillside seepages. It is unique in its combination of great height, aristate scales, achenes with relatively long stipes, and relatively low and faint transverse ridges on the faces. Vulnerable status appears warranted for R. belizensis because only two populations and fewer than 500 individuals are known, all within a small geographic area. DNA sequence data from the plastid marker trnL-trnF and nuclear markers ETS and ITS diagnose a clade of R. belizensis, its sister R. emaciata, and R. junciformis and R. saxisavannicola. Rhynchospora belizensis is the only member of this clade occurring in Central America; all the others are endemic to South America.
