We describe a new species of the papilionoid legume genus Aeschynomene (Leguminosae, Papilionoideae) that is tightly ecologically associated with Amazonian campos rupestres on iron-rich canga soils of the Serra dos Carajás mountain range, Pará, Brazil. The new species is morphologically similar to Aeschynomene rudis, but it differs by the smaller leaflets, larger flower, glabrous ovary, and a longer and sigmoid fruit stipe. We present a complete description, taxonomic comments, illustrations, and a distribution map for the new species. Additionally, we provide its phylogenetic position and an identification key to all Aeschynomene and Ctenodon species occurring on the canga of the Serra dos Carajás.