Hercegovina (also spelled Herzegovina), centered on the city of Mostar, is established as a duchy separate from Bosnia in 1448, when, in a document sent to Holy Roman Emperor Frederick III on January 20, 1448, Stephen Vukčić Kosača styles himself "Duke of Saint Sava (Vojvoda Svetog Save), lord of Hum and Primorje (Gospodar Humski i Primorski), Grand Duke (Herzog), and forces Bosnia to recognize him as such.

Hercegovina’s name derives from the German Herzog ("duke"), the title borne by its rulers.