The Duchy of Saint Sava is a late medieval monarchy amid the Ottoman conquest of the Balkans.

It is ruled by Stefan and his son Vladislav, of the Kosača family.The first ruler is titled "Voivode of Saint Sava" (after the first Serbian Patriarch, Saint Sava), his rank in German - Herzog (Duke), will later give the name to the present-day region of Herzegovina, thus it has also been named the "Duchy of Herzegovina" in modern sources, as the Ottomans used Hersek Sancağı ("Sanjak of the lands of the Herzog"), the Sanjak of Herzegovina.