Siemowit IV (also known as Ziemowit IV) (ca. 1353/1356 – 21 January 1426) was one of the Dukes of Masovia of the old Polish Piast royal family. His domain included the lands of Czersk, Rawa, Sochaczew, Płock and Gostynin. In 1381 he inherited Wisz and in 1387 Bełz.

Siemowit IV was the younger son of Siemowit III of Piast Dynasty and the younger brother to duke Janusz I of Warsaw. In 1381 brothers divided their father's realm between themselves thus de facto creating two Masovian duchies and Siemowit attempted to play international politics, refusing to become the vassal of the new king Louis I of Hungary while his brother followed his father's approach. He minted his own coin and attempted to influence the election of archbishop of Poland. Because of his meddling in the Crown politics his Masovian territories were subject of a ravaging summer raid, Polish and Hungarian forces, by young Sigismund of Luxemburg, then Margrave of Brandenburg, sent there by his father-in-law Louis I of Hungary and Poland in 1382. After the death of king Louis I of Hungary and Poland duke Siemowit, having found support in Greater Poland, become de facto one of the pretenders to the Polish throne. He took part in the civil war that took part in Greater Poland, raging between the two powerful Polish clans: Nałecze (Nałęcz coat-of-arms) clan and the Grzymalici ( Grzymała coat-of-arms) clan during 1382-85. In 1383 Siemowit conquered Cuyavia, but was soon expelled by the joint forces of szlachta from Lesser Poland and armies of Hungary. In 1386 he was forced to accept the rule of Queen Jadwiga and her husband Jogaila and became a hereditary vassal of Poland. The following year he married Jogaila's sister, Alexandra of Lithuania, becoming his brother-in-law and received land of Bełz.

Duke Siemowit, being ambitious, was very active in politics of the period and thus needed cash to finance his policies so several times he mortgaged some of his domains to the Teutonic Order, including Wizna, Zawkrze and Płońsk. In order to preserve his independence, albeit having sworn vassalage to the King of Poland, he tried to play his neighbours to his advantage, but was forced to take part in the war of 1409–1410 between Poland, Lithuania and the Teutonic Order although he was no enemy of the Order. During the Battle of Grunwald he fielded two banners of his own troops under his son Siemovit V, that fought alongside the 'Royal' Poles and Lithuanian troops. His banners had white eagle without a crown on a red filed as their badge according to Banderia apud Grunwald.

Issue

Alexandra bore Siemowit IV 13 children, to wit, 5 sons:

• Kazimierz II of Masovia
• Trojden II of Masovia
• Władysław I of Płock
• Siemowit V of Masovia
• Alexander of Masovia; who was a diplomat and Bishop of Trento, titular Bishop of Chur, titular Cardinal of Damascus, and Patriarch of Aquileia.

And 8 daughters:

• Euphemia of Masovia
• Cymburgis of Masovia
• Jadwiga of Masovia
• Amelia of Masovia; married William II, Margrave of Meissen
• Anna of Masovia
• Maria of Masovia; married Bogislaw IX, Duke of Pomerania
• Alexandra of Masovia
• Katarzyna of Masovia

Alexandra's and Siemowit's daughter Cymburgis married Ernest, Duke of Austria in 1412. The wedding took place in Buda (German: Ofen), at the residence of Sigismund, Holy Roman Emperor. At the time the emperor mediated negotiations between Alexandra's brother Władysław II Jagiełło and the Teutonic Knights. Two other daughters married Michael Žygimantaitis, pretender to the throne of the Grand Duchy of Lithuania. Grandchildren of Alexandra and Siemowit IV included Frederick III, Holy Roman Emperor, Przemyslaus II, Duke of Cieszyn, Sophie of Pomerania, Duchess of Pomerania and Dorothy Garai, queen of Bosnia.