Derris rubricosta Boonprajan & Sirich., sp. nov., a new species of the genus Derris Lour. (Fabaceae) was discovered in Peninsular Thailand. The overall morphology demonstrates that the species most resembles D. pubipetala. Nevertheless, the species has several autapomorphies differentiating it from other Derris species, e.g., the presence of reddish midribs of the mature leaflets, sparsely hairy stamen filaments, prominent hairs at the base of the anthers, and presence of glandular trichomes along the leaflet midrib. Additionally, HPLC fingerprints of this species showed a distinction from D. pubipetala by the absence of phytochemical compound peaks after 13 min. Retention Time (RT). Results from molecular phylogenetic analyses also strongly supported the taxonomic status as a new species.