Hedysarum alamutense, a new species in the tribe Hedysareae DC. (Fabaceae), is described and illustrated. It belongs to the traditionally recognized Hedysarum L. section Multicaulia (Boiss.) B.Fedtsch., which extends over the West Alborz Mountains in northern Iran. This species is characterized by greenish stems and corolla persisting in fruiting stage and has mostly 1?2-jointed, unarmed, biconvex pods with short hairs becoming bald early, and flattened margins. Phylogenetic analyses of molecular data clearly unite this species with H. formosum Fisch. & C.A.Mey. ex Basin. The new species is characterized by 4 singleton nucleotide substitutions, suggesting it as a distinct taxon. Phylogenetic inferences are consistent with the interpretations of morphological features. Moreover, the lack of nucleotide variation among individual samples of H. formosum indicated that the species has not been diversified at the population level and reinforced our findings.