Unifoliolate leaves are relatively rare in the species-rich dalbergioid clade of papilionoid legumes (Leguminosae, Papilionoideae). For the pantropical, diverse genus Aeschynomene (ca. 180 species), the only known record of unifoliolate species described so far is the savanna-inhabiting A. simplicifolia from Chapada dos Veadeiros in Goiás, Brazil. The current work provides a description, illustrations, and a distribution map of another unifoliolate species, Aeschynomene chicocesariana, which was newly discovered in the rupestrian grasslands of Chapada Diamantina in Bahia, Brazil. The new species readily differs from A. simplicifolia by having mostly single-stemmed, erect habit, leaves with considerably shorter, 1.5–4 mm long petioles, leaflet blades densely pubescent beneath and glabrescent above and inconspicuously veined on both surfaces and without a marginal vein, and much smaller, ca. 15 × 14 mm standard petal. Bayesian phylogenetic analyses of DNA sequence data (nuclear ribosomal ITS/5.8S and the plastid genes matK and trnL intron) have unequivocally placed A. chicocesariana with the rest of Aeschynomene species that are traditionally placed within sect. Ochopodium. This result concurs with the shared morphology of A. chicocesariana and members of that section.