| Native Name: | Księstwo czerskie (Polish) Ducatus Cirnensis (Latin) |
| Conventional Long Name: | Duchy of Czersk |
| Era: | High Middle Ages |
| Status: | Independent state (1275–1294) Fiefdom within the Kingdom of Poland (1310–1320) Fiefdom of the United Kingdom of Poland (1320–1370) Fiefdom of the Crown of the Kingdom of Poland (1471–1495) |
| Government Type: | Feudal duchy |
| Life Span: | 1275–1294 1310–1370 1471–1495 |
| Event Start: | Partition of the Duchy of Masovia |
| Date Start: | 1275 |
| Event1: | Unification of the Duchy of Masovia |
| Date Event1: | 24 June 1294 |
| Event2: | Partition of the Duchy of Masovia |
| Date Event2: | 1310 |
| Event3: | Unification of the Duchy of Masovia |
| Date Event3: | 5 November 1370 |
| Event4: | Partition of the duchies of Płock and Warsaw |
| Date Event4: | 1471 |
| Event End: | Reformation into the Duchy of Masovia |
| Date End: | 1495 |
| P1: | Duchy of Masovia |
| Flag P1: | Chorągiew księcia mazowieckiego Siemowita.svg |
| Border P1: | no |
| P2: | Duchy of Płock |
| Flag P2: | Chorągiew księcia mazowieckiego Siemowita.svg |
| Border P2: | no |
| P3: | Duchy of Warsaw (Middle Ages)Duchy of Warsaw |
| Flag P3: | Chorągiew Mazowsza.svg |
| Border P3: | no |
| S1: | Duchy of Masovia |
| Flag S1: | Chorągiew księcia mazowieckiego Siemowita.svg |
| Border S1: | no |
| Religion: | Roman Catholic |
| Alt Coat: | Coat of arms |
| Image Map Caption: | The map of the political division of Masovia between 1313 and 1345, including the Duchy of Czersk. |
| Capital: | Czersk |
| Official Languages: | Polish, Latin |
| Title Leader: | Duke |
| Leader1: | Konrad II of Masovia |
| Year Leader1: | 1275–1294 (first state) |
| Leader2: | Trojden I |
| Year Leader2: | 1310–1341 (first of the second state) |
| Leader3: | Siemowit III |
| Year Leader3: | 1355–1370 (last of the second state) |
| Leader4: | Konrad III Rudy |
| Year Leader4: | 1471–1495 (third state) |
The Duchy of Czersk was a feudal district duchy in Masovia, centered on the Czersk Land. Its capital was Czersk.
The country was established in 1275, in the partition of the Duchy of Masovia, with duke Konrad II becoming its ruler.[1] After his death, the duchy was unified with the Duchy of Płock, forming the Duchy of Masovia, on 24 June 1294.[2] The state was again reestablished in 1310, with Trojden I, as its first ruler.[3] It existed until 5 November 1370, when, under the rule of duke Siemowit III, duchies of Czersk, Rawa, and Warsaw were unified into the Duchy of Masovia.[4] [5] It was once again reestablished in 1471, from the part of the territories of the duchies of Płock and Warsaw. In 1488, it incorporated the Duchy of Warsaw, into its territory.[6] It existed until 1495, when, with the incorporation of the Duchy of Płock into the Kingdom of Poland, it remained the only state in Masovia, and subsequently, got reformed into the Duchy of Masovia.[7]
Between 1275 and 1294, it was an independent state, while from 1310 to 1320, it was a fiefdom within the Kingdom of Poland, and from 1320 to 1370 a fiefdom of the United Kingdom of Poland, and from 1471 to 1495, a fiefdom of the Crown of the Kingdom of Poland.