Boleslaw I of Masovian (Polish: Bolesław I Mazowiecki) (1208 – spring 1248) was a member of the House of Piast. He was the eldest son of Konrad I of Masovia and his wife Agafia of Rus.
He was the Duke of Sandomierz (he reigned over part of the duchy) from 1229 to 1232. In 1233 he became the Duke Northern Masovia and in 1247 he inherited all of the Duchy of Masovia.
From a young age, Boleslaw was part of ambitious political projects to do with his father. In 1227 after the assassination of Leszek I the White, he accompanied his father to Lesser Poland to participate in negotiations on the succession. Władysław III Spindleshanks had plans to seize the Duchy of Cracow from Masovia. Despite this setback, Konrad gave his son the stronghold of Dobrzyń nad Wisłą. In 1229 Konrad took a part of the Duchy of Sandomierz with the city Radom, Konrad offers this in exchange for Boleslaw to give Dobrzyń nad Wisłą to Konrad's other son, Siemowit. In 1231, Boleslaw became involved with his father and brothers in the installation of the Teutonic in the region of Chełmno. At the same time, Boleslaw supported his father in the war to conquer Cracow, after the death of Ladislas III. In 1232 Henry I the Bearded invades Masovia by Prussians forced to abandon Konrad. In addition, Henry I the Bearded assumes control of the entire Duchy of Sandomierz.
To compensate for the loss of the Duchy of Sandomierz, Konrad offered Boleslaw part of Masovia, with Sieradz, the capital. Nevertheless, Konrad retained control of foreign policies and the army. In 1234, Boleslaw won the north of Masovia in exchange for the region of Sieradz, near Lesser Poland and strategically important for his father who aimed to become Duke of Kraków. In subsequent years, Boleslaw attacked his Prussian and Russian neighbors. He installs the Order of Dobrin to Drohiczyn. In 1238 he took part in the war against Daniel of Galicia and Mindaugas of Lithuania.
In 1241, after the death of his father-in-law Henry II the Pious at the Battle of Legnica, Boleslaw maintained his father's military seizes on Kraków. He was expelled two years later.
On 31 August 1247, Konrad died, this opened new horizons for Boleslaw. In Konrad's will, he receives most of Masovia. He faced an assault from his brother Casimir I of Kuyavia because Casimie was unhappy hiw much Boleslaw had inherited. Casimir chased after his brothers, Boleslaw and Siemowit and Dobrzyńnad nad Wisla.
Boleslaw died shortly after the spring 1248. In his will he bequeathed all his possessions to his younger brother Siemowit. This however annoyed Casimir and so there were wars to come between the brothers [1].
Boleslaw I of Masovia married twice. His first marriage was to Gertrude, daughter of Henry II the Pious in 1234. Gertrude died ten years later, the couple had no children. Two years later in 1246, Boleslaw married Anastasia, daughter of Prince Alexander of Belz. This marriage was also childless.
| Preceded by Konrad I |
Duke of Masovia 1247–1248 |
Succeeded by Siemowit I |