Gradyana franciscana, a new genus and species of Euphorbiaceae found in the São Francisco river valley, in the semiarid region of northeastern Brazil, is herein described and illustrated. Gradyana is morphologically close to Stillingia due to the presence of carpidiophore and foliar glands, but differs in possessing strictly axillary sinuous thyrsoid staminate inflorescences, solitary pistillate flowers, apical glands in the bracts, long-stipitate glands between pistillate sepals, and staminate flowers with three stamens and three sepals. A key to differentiate Gradyana from other genera of the tribe Hippomaneae occurring in South America is presented. Comments about its geographic distribution, habitat, and conservation status are also provided.