| Native Name: | Księstwo sieradzkie (pl) ducatus Siradiae (la) |
| Conventional Long Name: | Duchy of Sieradz Duchy of Siradia |
| Common Name: | Sieradz |
| Era: | Middle Ages |
| Status: | Province of Poland Fiefdom of the Polish Crown (from 1306) |
| Year Start: | 1263 |
| Year End: | 1339 |
| Event1: | Ruled by Bohemia |
| Date Event1: | 1299 |
| Event2: | Vassalized by the Polish Crown |
| Date Event2: | 1305 |
| Event End: | Incorporated by Poland |
| P1: | Duchy of Kuyavia |
| Flag P1: | POL województwo brzeskokujawskie IRP COA.svg |
| Border P1: | no |
| S1: | Sieradz Voivodeship (1339–1793)Sieradz Voivodeship |
| Flag S1: | POL województwo sieradzkie IRP COA.svg |
| Border S1: | no |
| Government Type: | Duchy |
| Religion: | Roman Catholic |
| Image Map Caption: | Poland 1275-1300, Duchy of Sieradz marked in violet |
| Capital: | Sieradz |
The Duchy of Sieradz (Latin: ducatus Siradiae, Polish: Księstwo Sieradzkie), also known as the Duchy of Siradia,[1] was created based on part of the Duchy of Łęczyca in 1263.[2] Duchy of Łęczyca was created around 1231 from Province of Łęczyca, a part of the Seniorate Province by Konrad I of Masovia.
Ruled by the rivaling Masovian branch of the Piast dynasty. In 1299 Duke Władysław I the Elbow-high had to cede Sieradz to King Wenceslaus II of Bohemia, who had also obtained the Seniorate Duchy of Kraków in 1291. Nevertheless, upon the extinction of the Bohemian Přemyslid dynasty in 1306, it was reunited with the Kingdom of Poland as a vassal duchy, and after 1339 incorporated by King Casimir III the Great into the Lands of the Polish Crown as Sieradz Voivodship. Around that time, the term Sieradz Land (Terra Siradiensis, ziemia sieradzka) begun replacing the older Duchy nomenclature.
The importance of the territory is reflected in the Latin title of Polish kings: nec non terrarum Cracovie, Sandomirrie, Lancicie, Cuyavie, Syradziensis dux.
pl:Marek Koter
. Miasto – region – gospodarka w badaniach geograficznych. W stulecie urodzin Profesora Ludwika Straszewicza . 2016 . . Tomasz Marszał (red.) . 9788380881228 . Łódź . 131–142 . pl . Historyczno-geograficzne podstawy oraz proces kształtowania się regionu łódzkiego . 2022-06-16 . https://dspace.uni.lodz.pl/xmlui/bitstream/handle/11089/18242/8-131_161-Koter.pdf#page=1.