| Siemowit I | |
|---|---|
 Siemowit I, Duke of Masovia by Jan Matejko | |
| Duke of Masovia | |
| Reign | 1248–1262 |
| Predecessor | Bolesław I of Masovia |
| Successor | Konrad II of Masovia |
| Died | 23 June 1262 |
| Spouse | Pereyaslava of Halych |
| Issue | Konrad II of Masovia Bolesław II of Masovia Salomea |
| House | House of Piast |
| Father | Konrad I of Masovia |
| Mother | Agafia of Rus |
Siemowit I of Masovia (Polish: Siemowit (Ziemowit) I mazowiecki) (d. 23 June 1262), was a Polish prince member of the House of Piast, Duke of Czersk during 1247–1248, Duke of Masovia (except Dobrzyń) during 1248–1262, ruler over Sieradz during 1259–1260.
He was the sixth son of Konrad I of Masovia and his wife Agafia of Rus.[1]
- ^ Rowell 1994, p. xxix.