Diplotaxis (Brassicaceae) is a genus widely distributed in the temperate areas of Europe, Asia and Africa, reaching its greatest diversity in NW Africa, Cape Verde and the Mediterranean area. In eastern Africa, this genus is only represented by D. harra (Forssk.) Boiss. in Somalia and Djibouti, while no native species of Diplotaxis have been recorded from Ethiopia. The finding of Diplotaxis kohlaanensis A.G. Mill. & J.A. Nyberg, originally described from northern Yemen, is here reported for the first time in the Simien Mountains. Our finding has a particular phytogeographical and ecological significance, highlighting the relevant floristic relationships of the Arabian Peninsula mountains with the Ethiopian highlands. In addition, our investigations confirm the outstanding number of exclusive taxa shared by these two areas testifying to the past phytogeographical connection between these two territories. D. kohlaanensis is localised on the high-altitude cliffs, which represent a highly conservative environment for relict species. Finally, the taxonomical relationships of this taxon within the Diplotaxis harra complex are also examined.