Ruppia mongolica (Ruppiaceae), a new species from Inner Mongolia (China), based on morphological and genetic data.

Y. Zou & X.W. Xu, a new species from Inner Mongolia, China, is described and illustrated. The phylogenetic position of the new species within the genus was analyzed based on eight chloroplast DNA fragments and an ingroup sampling of all Eurasian species of . The results showed that formed a separate branch between and the clade of , , , and . Based on molecular and geographical evidence, our study reveals that is closely related to and but differs from the former in the length and shape of the peduncle and seed size, and from the latter in the length of the peduncle, number of carpels per flower, and seed size. In addition, the karyotype analysis revealed that is octoploid, which is first reported within , further supporting as a new species.