Blache’s flounder, Arnoglossus blachei Stauch, 1965, is redescribed from the eastern Atlantic (Morocco south to Namibia, including the Canary Islands, larvae reaching South Africa) and distinguished from the imperial flounder A. imperialis (Rafinesque, 1810) by having a live colouration of the lower side of males which is bright red (whitish in A. imperialis), the caudal fin with lateral-line scales 52-57 (vs. 58-63), and the dorsal-fin filaments in male light grey, with black margins, distally yellowish (vs. plain white). A lectotype is selected for Arnoglossus blachei. The species is compared with similar species. The externally similar species Arnoglossus imperialis is also redescribed; its distibution range is restricted to the Mediterranean and northeastern Atlantic, including Madeira and the Azores. Neotypes are selected to stabilize the usage of the names Bothus imperialis Rafinesque, 1810, Bothus punctatus Rafinesque, 1814, Rhombus cristatus Lowe, 1839 and Charybdia rhomdoidichthys Facciolà, 1885. Molecular analyses based on two mitochondrial fragments (12s rRNA and COI) clearly support the validity of A. blachei. Moreover, though A. blachei is morphologically close to A. imperialis, the two species show the highest genetic distances among all Arnoglossus species compared (including A. capensis, A. imperialis, A. laterna, A. rueppelii and A. thori). A key to the eastern Atlantic and Mediterranean species of Arnoglossus is presented.