Rhynchospora megaplumosa is described as a new species of Rhynchospora, endemic to central Florida, in the section Rhynchospora series Plumosae. It is restricted to sandy openings in scrubby flatwoods in two localized areas of central Florida, in Polk and Manatee counties, where it flowers profusely in areas following burning. Rhynchospora megaplumosa is most closely related to R. pineticola and R. plumosa, but is distinguished by several characters, most conspicuously by its longer (5–7 mm) perianth bristles and golden-brown, narrowly lanceolate, longer spikelets. A key is provided to distinguish it from other species in the series. The habitat and associated species of R. megaplumosa, are also discussed.