Siemowit III, Duke of Masovia

Siemowit III of Masovia
Princely seal of Siemowit III; 1371
Spouse(s)	Euphemia of Opava Anna of Ziębice
Noble family	House of Piast
Father	Trojden I of Masovia
Mother	Maria of Galicia
Born	c. 1320
Died	16 June 1381(1381-06-16)
Buried	Plock cathedral

Siemowit III of Masovia (his name also rendered Ziemowit; c. 1320 – 1381) was a prince of Masovia and a co-regent (with his brother Casimir I of Warsaw) of the lands of Warsaw, Czersk, Rawa, Gostynin and other parts of Masovia.

Siemowit was the second son of Trojden I of Masovia and his wife Maria, daughter of Yuri I of Galicia.

In 1341, following the death of their father and brother, Siemowit and his younger brother Casimir inherited the Duchy of Czersk. In 1345 following the death of their uncle Siemowit II of Rawa, they also inherited the Duchy of Rawa. In 1349 the two brothers shared their possessions. Siemowit gave the small region of Warsaw to his brother Casimir, retaining the regions of Czersk, Liw and Rawa.

Early in his reign, Siemowit tried to establish good neighborly relations with his powerful neighbors: the Teutonic Order, Poland and Bohemia. Some historians believe that Siemowit rendered a tribute of vassalage to Charles IV, Holy Roman Emperor in 1346. Other historians say this happened in 1351, in order to inherit from Boleslaw III of Płock who was a vassal of Bohemia.

But on September 18, 1351, Siemowit and Casimir recognised the Polish King Casimir III the Great as suzerain, canceling the allegiance of Masovia and Bohemia. In exchange, they got Gostynin (Siemowit) and Sochaczew (Casimir), former possessions of Boleslaw III. Casimir also gave the Duchy of Płock to the brothers.