A newly naturalized plant, Neptunia plena (L.) Benth. (Leguminosae), native to America and Asia, has recently been found in low elevations of southern Taiwan. Neptunia plena differs from the other Taiwanese Neptunia spp. in erect stem, bracts on the lower half of the peduncle and with a suppressed gland between or just below the lowest pair of pinnae. A detailed description, line-drawings, photographs and geographic distribution are provided for identification of this species.