Derris erythrocosta Boonprajan & Sirich., sp. nov., a new species of the genus Derris Lour. (Fabaceae) was discovered from Peninsular, Thailand. The taxon was considered as a distinct species in terms of integrative taxonomy approach. The overall morphology demonstrated that this species most resembled D. pubipetala. According to the macro-morphological and leaves micro-morphological studies, this taxon has several autapomorphies distinctively different from other Derris species, e.g., the presence of reddish midribs of the mature leaflets, sparsely hairy filaments, prominent hairs at base of anthers, and presence of glandular trichomes along the midrib. Additionally, HPLC fingerprints from phytochemical study of this species were also distinct. Results from molecular phylogenetic analyses strongly supported and clearly confirmed its status as a new of the genus Derris.