A new species of Cyanus Miller, C. diospolitanus Bancheva & S. Stoyanov (Asteraceae, Centaureinae), from Tundzha hilly country floristic region in Elhovo municipality, southeastern Bulgaria, is described and illustrated. It belongs to Cyanus sect. Napuliferi (Stefanoff & T. Georgiev) Bancheva & Raimondo, and taxonomically its closest relative is C. pseudoaxillaris (Stefanoff & T. Georgiev) Holub, a rare local endemic for the Thracian Plain, Bulgaria. The new species is currently known from four localities in the Derventski Hills (Yambol Province), growing at the periphery of dry, stony meadows in thermophilous oak forests and brushwood. The chromosome number is 2n  =  22, and SEM analysis of the pollen grains is described and illustrated.