The Duchy of Żagań or Duchy of Sagan was one of the duchies of Silesia ruled by the Silesian Piasts. Its capital was Żagań in Lower Silesia, the territory stretched to the town of Nowogród Bobrzański in the north and reached the Lusatian Neisse at Przewóz in the west, including two villages beyond the river.

The Duchies of Silesia were the more than twenty divisions of the region of Silesia formed between the 12th and 14th centuries by the breakup of the Duchy of Silesia, then part of the Kingdom of Poland. In 1335, the duchies were ceded to the Kingdom of Bohemia under the Treaty of Trentschin. Thereafter until 1742, Silesia was one of the Bohemian crown lands and lay within the Holy Roman Empire. Most of Silesia was annexed by the King of Prussia under the Treaty of Berlin in 1742. Only the Duchy of Teschen, the Duchy of Troppau and the Duchy of Nysa remained under the control of the Bohemian crown and as such were known as the Duchy of Upper and Lower Silesia until 1918.