Cyperus pacificus (Ohwi) Ohwi var. margoinflatus S. Fujii, a new variety of C. pacificus (Cyperaceae) is described. It closely resembles C. pacificus var. pacificus in the oblong-ellipsoid achenes with winged margins, but the margins are well inflated with spongy parenchyma. It usually occurs in the dried bottoms of irrigation ponds, and often intermixed with other species of Cyperaceae such as C. extremiorientalis Ohwi, C. nipponicus Franch. et Sav. var. nipponicus and var. spiralis Ohwi, Fimbristylis dipsacea (Rottb.) C.B. Clarke, and F. squarrosa Vahl var. esquarrosa Makino. A key to the new variety and closely related taxa is also provided.