Siemowit I of Masovia

Siemowit I (c. 1215–1262) was a Piast prince, a son of Konrad I of Masovia. He inherited Masovia from his brother Bolesław I in 1248. To counter Baltic raiders he allied with Daniel of Halych, married his daughter, joined campaigns against the Yotvingians and backed the Teutonic Order.

In 1262 Lithuania and Rus' invaded under Mindaugas. On 23 June 1262 the fortress of Jazdów fell through betrayal and Siemowit was killed, while his son Konrad II was taken captive.