Siemowit IV (also known as Ziemowit IV) (ca. 1352–1425 or 1426) was one of the Dukes of Masovia. His domain included the lands of Czersk, Rawa, Sochaczew, Płock and Gostynin. In 1381 he inherited Wisz and in 1387 Bełz. Siemowit IV was a son of Siemowit III. After the death of Ludwik Węgierski he was one of the pretenders to the Polish throne. In 1383 he conquered Cuyavia, but was soon expelled by the joint forces of szlachta from Lesser Poland and armies of Hungary. In 1386 he accepted the rule of Grand Duke of Lithuania and King of Poland Jogaila and became a hereditary vassal of Poland. The following year he married the sister of the Polish king, Aleksandra, and received the land of Bełz.
He lost much of his domain to the Teutonic Order, including Wizna, Zawkrze and Płońsk. To get it back, he took part in the war of 1409–1410 between Poland, Lithuania and the Teutonic Order. During the Battle of Grunwald he fielded three banners of his own troops, fighting alongside the Poles.