The Duchy of Prussia or Ducal Prussia is a duchy in the eastern part of Prussia from 1525–1701.

It is 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 is "Prut(h)enic".In 1525, during the Protestant Reformation, the Grand Master of the Teutonic Knights, Albert, secularizes the order's Prussian territory, becoming Albert, Duke of Prussia.

His duchy, which has its capital in Königsberg (Polish: Królewiec), is established as fief of the Crown of Poland.

It is 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, achieves full sovereignty over the territory in the 1657 Treaty of Wehlau, which is confirmed in the 1660 Treaty of Oliva.

The Duchy of Prussia is elevated to the Kingdom of Prussia in 1701.