Abstract: Polygonum jucundum var. rotundum Z. Z. Zhou & Q. Y. Sun, a new variety of Polygonum (Polygonaceae) from Shengjin Lake, southeastern Anhui, China, is described and illustrated. The variety differs from P. jucundum var. jucundum Meisn. by having leaf blade linear-lanceolate, 4.5–9.0×0.5–0.9 cm, base rounded, petiole nearly absent, and stems 1.5–2.0 mm in diam. The new variety is similar to P. longisetum var. rotundatum A. J. Li in leaf shape, but differs by plants taller, up to 160 cm, rooting at 20–26 base nodes, adventitious roots 10–16 cm long, pedicels longer than bracts, 4–5 mm long. The pollen morphology, tepal and fruit microcharacteristics of the new varie