Koenigia chuanzangensis Z. Z. Zhou & Y. J. Min (Polygonaceae) from southeastern Xizang and western Sichuan Provinces, China, is described and illustrated. The new species morphologically resembles K. islandica L. and K. fertilis Maxim., sharing a habit of dwarf annual herbs, the leaf margins entire, pollen echinate and pantoporate, and the achene slightly exceeding a persistent perianth and brown, dull, narrowly ovoid, and biconvex. Koenigia chuanzangensis differs mainly in the pilose indument of its four tepals, tender stems, and leaves. Plants of the new species are usually 1–3 cm tall, commonly as separate plants, and leaf blades are ovate to elliptic and often as several fascicles at branch apices. In contrast, K. islandica and K. fertilis have glabrous plants that are taller, with glabrous adaxial leaf blade surfaces, and tepals that are especially glabrous. Stems of both species are tufted, with alternate leaves and leaf blades that are obovate.