ABSTRACT A new species of Grewia L. endemic to the Sambirano phytogeographic region of northwestern Madagascar is described. Specimens of this species were annotated by Rene Capuron over 40 years ago as “Grewia hispidissima”, which he evidently believed represented a species new to science, but the name has not been validly published. Our taxonomic work on the genus in Madagascar, including a careful study of Capuron's annotated specimens and a wealth of new material, has confirmed that this plant deserves recognition at the species level. A suite of characters clearly distinguishes Grewia hispidissima Wahlert, Phillipson & Mabb., sp. nov. from all other species in Madagascar, including its large acuminate leaves and the mix of simple and stellate ferruginous trichomes that densely cover its stems, petioles, inflorescence axes, pedicels and sepals. Illustrations, a distribution map, and a preliminary conservation assessment using the IUCN Red List criteria are provided for the new species.