| Native Name: | Ducatus Sandomirensis (Latin) Księstwo sandomierskie (Polish) |
| Conventional Long Name: | Duchy of Sandomierz |
| Era: | High Middle Ages |
| Status: | Fiefdom of Kingdom of Poland |
| Government Type: | District principality |
| Life Span: | 1138–1320 |
| Event Start: | Testament of Bolesław III Wrymouth |
| Date Start: | 1138 |
| Event End: | Incorporation into Kingdom of Poland |
| Date End: | 1320 |
| P1: | Kingdom of Poland (1025–1385)Kingdom of Poland |
| Flag P1: | Flag of the Kingdom of Poland.svg |
| Border P1: | no |
| S1: | Kingdom of Poland (1025–1385)Kingdom of Poland |
| Flag S1: | Flag of the Kingdom of Poland.svg |
| Border S1: | no |
| S2: | Duchy of Wiślica |
| Religion: | Roman Catholic |
| Image Map Caption: | Kingdom of Poland between 1304 and 1333, including Duchy of Sandomierz. |
| Capital: | Sandomierz |
| Official Languages: | Polish, Latin |
| Title Leader: | Duke |
| Leader1: | Władysław II the Exile |
| Year Leader1: | 1138–1146 (first, regency) |
| Leader2: | Henry of Sandomierz |
| Year Leader2: | 1146–1166 (first proper) |
| Leader3: | Władysław I Łokietek |
| Year Leader3: | 1304–1320 (last) |
The Duchy of Sandomierz was a district principality and a fiefdom of the Kingdom of Poland. It was formed in 1138 from the territories of the Kingdom of Poland, following its fragmentation, that was started by the testament of Bolesław III Wrymouth. The Duchy existed until 1320 when it was incorporated into the Kingdom of Poland. Its capital was the city of Sandomierz. The territory was centred around Sandomierz Land in the southeastern corner of Poland, between the rivers of Pilica, Vistula, San, and Dunajec.
The period of fragmentation of Poland began in 1138, after the Testament of Bolesław III Krzywousty. According to the testament, Duke Henry of Sandomierz was granted the Land of Sandomierz. Since the duke was too young, the newly created province was temporarily ruled by High Duke Władysław II the Exile. In 1146, after another conflict with the junior dukes, Wladyslaw II was forced to flee abroad, and the Duchy of Sandomierz was transferred to Henry.
Henry died in 1166, killed by the Old Prussians. Since he was childless and had no wife, part of his province, with Wislica, was inherited by his youngest brother, Casimir II the Just, while Sandomierz was seized by Bolesław IV the Curly. The province remained divided until 1173, when Casimir II the Just reunited it. Casimir II died in 1194, and the Duchy of Sandomierz was inherited by Leszek I the White, and his younger brother, Konrad I of Masovia. In c. 1200 the brothers divided their properties, with Konrad receiving Mazovia and Kujawy, while Leszek remained in Sandomierz. In 1205, the two brothers united their forces, defeating Roman the Great in the Battle of Zawichost.
In the 13th century, the Duchy of Sandomierz was several times raided by the Mongol hordes (see, e.g., Sack of Sandomierz (1241) and Sack of Sandomierz (1260)). In the late 1280s, Duke Władysław I the Elbow-high took control over Sandomierz, but in 1292, he was forced by Wenceslaus II of Bohemia to abandon the province. In 1304, Lokietek returned, capturing Wislica, and after the death of Wenceslaus (1305), he seized the whole province. During the reign of Lokietek, the Duchy of Sandomierz was turned into Sandomierz Voivodeship.
The Duchy of Sandomierz had twelve castellanies, located in the following towns: Łuków, Lublin, Sieciechów, Radom, Skrzynno, Żarnów, Małogoszcz, Czchów, Wislica, Połaniec, Zawichost and Sandomierz.