Inga kursarii is a new species collected in the terra firme forests of the northwestern Amazon, one of the regions with the highest tree species diversity in the world. According to morphological and phylogenetic analyses, the new species is morphologically similar and sister to Inga gracilifolia Ducke, but it can be distinguished by having 5–6 pairs of caducous leaflets, elliptical leaflets with acute apex and slightly asymmetrical base, spiked inflorescence, subsessile flowers, calyx tube with 4 lobes, tufts of hairs at the apex of calyx lobes, corolla tube with non-reflexed lobes, shorter staminal tubes, and capitate stigma. In addition, analyses of the chemical defensive profile (chemocoding) show that both taxa are chemically different, with I. kursarii having a chemistry based on gallocatechin/epigallocatechin gallates, and I. gracilifolia producing a series of dihydroflavonols. Finally, we present a table with a comparison of diagnostic characters that allows separation of the two species.