A new species, Pseudostellaria polymorpha Y. S. Lian (Caryophyllaceae), is described from Gansu Province, China. The species is remarkable for the regular variation in its floral morphology from stem apex to base. The terminal chasmogamous flower is largest, having five oval-lanceolate sepals, five oblanceolate petals noticeably longer than the sepals, 10 stamens, and an ovary with two long filamentous styles. The flowers along the upper portions of the stems have four sepals, only two or three petals equal or slightly shorter than the sepals, six to eight stamens including one or two sterile ones, and an ovary with two filamentous, shorter styles. The cleistogamous flowers are solitary in the axils of the basal portion of the stem, with four densely pilose sepals, three petals obviously shorter than the sepals or reduced, two sterile stamens, and an ovary with two styles obviously shorter than the ovary itself. This floral transition is rare in Pseudostellaria Pax, and is therefore a feature by which the new species is easily distinguishable from other members of the genus.