The new species Rhynchosia franciscana (Leguminosae, Papilionoideae) is described and illustrated from the seasonally dry vegetation on sand dunes of the middle São Francisco River in the state of Bahia, Brazil. The new species is included in Rhynchosia section Copisma, in which it can be differentiated from other species by its relatively larger flowers and wing petals that are shorter than the keel. The discovery of this narrowly endemic new plant species adds to the spectacularly rich biodiversity that has been revealed in the Caatinga seasonally dry woodlands in the middle São Francisco River valley over the last 30 yr.