A duchy, also called a dukedom, is a medieval country, territory, fief, or domain ruled by a duke or duchess, a ruler hierarchically second to the king or queen in Western European tradition.

There once existed an important difference between "sovereign dukes" and dukes who were ordinary noblemen throughout Europe. Some historic duchies were sovereign in areas that would become part of nation-states only during the modern era, such as happened in Germany (once a federal empire) and Italy (previously a unified kingdom). In contrast, others were subordinate districts of those kingdoms that had unified either partially or completely during the medieval era, such as France, Spain, Sicily, Naples, and the Papal States.