The Duchy of Brzeg (Polish: Księstwo Brzeskie) or Duchy of Brieg (German: Herzogtum Brieg), (Czech: Knížectví Břežské) was one of the Duchies of Silesia. Its capital was Brzeg.
It was created in 1311 during the fragmentation of the Duchy of Legnica among the sons of Duke Henry V and ruled by Bolesław III the Generous of the Silesian Piasts, who declared himself a vassal of King John of Bohemia in 1329. It was reunited with the Duchy of Liegnitz in 1419, then fragmented again, and united once more with Legnica under Duke Christian in 1664. When the Kingdom of Bohemia was inherited by the Habsburg dynasty of Austria in 1526, the duchy fell under their overlordship as Bohemian kings, although it was still ruled as a lien by the Silesian Piasts.
In 1537 Duke Frederick II concluded a treaty with Elector Joachim II Hector of Brandenburg, whereby the Hohenzollern dynasty would inherit the duchy upon the extinction of the Silesian Piasts. This agreement however was rejected by the Bohemian king Ferdinand I of Habsburg and did not come into effect.
Together with Legnica, Brzeg was the last autonomous duchy of Silesia. Following the death of the last Piast duke George William in 1675, it was administered directly by the House of Habsburg in dynasty's capacity as Kings of Bohemia. However Brandenburg-Prussia also claimed the duchy, referring to the old inheritance treaty of 1537. Nevertheless the Habsburg Monarchy again refused to acknowledge the validity of this agreement and annexed the duchies. Several decades later King Frederick II of Prussia used the dispute as a pretext to justify his campaign during the First Silesian War in 1740.