A new species of Onosma L. (Boraginaceae: Lithospermeae), from the Hakkari province of eastern Turkey, is described and illustrated. The new species is endemic and distributed in a narrow area in the Sat Mountains. Taxonomically the closest relative of Onosma satensis sp. nov. is O. polioxantha Rech. f. which also occurs in Turkey. In addition, it is primarily distinguished from O. polioxantha by its navicular leaves, longer and reticulate cauline leaves, longer pedicels and calyx, longer and densely patent short setulose outside, reticulate corolla and bigger nutlets. The geographical distributions and map, detailed photographs, habitat description, vernacular name, an IUCN conservation status and identification key for O. satensis sp. nov. and O. polioxantha were also provided. Nutlets and pollen micrographs of O. satensis sp. nov. are presented using scanning electron microscopy.