The Duchy of Prussia or Ducal Prussia (; ; ; ; Old Prussian: Ercōgista Prūsija ) was a duchy in the eastern part of Prussia from 1525–1701. It was the first Protestant (Lutheran) duchy with a dominant German-speaking population, as well as Polish and Lithuanian minorities. In old texts and in Latin, the term Prut(h)enia refers alike to Ducal Prussia, its western neighbor Royal Prussia, and their common predecessor, Teutonic Prussia. The adjectival form of the name was "Prut(h)enic".
In 1525 during the Protestant Reformation, the Grand Master of the Teutonic Knights, Albert, secularized the order's Prussian territory, becoming Albert, Duke of Prussia. His duchy, which had its capital in Königsberg (Polish: Królewiec), was established as fief of the Crown of Poland. It was inherited by the Hohenzollern prince-electors of Brandenburg in 1618; this personal union is referred to as Brandenburg-Prussia. Frederick William, the "Great Elector" of Brandenburg, achieved full sovereignty over the territory in the 1657 Treaty of Wehlau, which was confirmed in the 1660 Treaty of Oliva. The Duchy of Prussia was elevated to the Kingdom of Prussia in 1701.