Konstantin (Kostadin) Dejanović (; 1365-95) was a Serbian magnate that ruled a large province in eastern Macedonia under Ottoman suzerainty, during the fall of the Serbian Empire. He succeeded his older brother Jovan Dragaš, who had been an Ottoman vassal since the Battle of Maritsa (1371) which had devastated part of the Serbian nobility. The brothers had their own government and minted coins according to the Nemanjić style. His daughter Jelena married Byzantine Emperor Manuel II Palaiologos in 1392. He fell at the Battle of Rovine (17 May 1395), serving the Ottomans against Wallachia, fighting alongside Serbian magnates Stefan Lazarević and Marko Mrnjavčević.