Siemowit III of Masovia

Siemowit III (c. 1320 – 16 June 1381) was a Piast duke of Masovia, the second son of Trojden I and Maria of Galicia. In 1341 he and his brother Casimir inherited Czersk, adding Rawa in 1345; in 1349 he kept Czersk, Liw and Rawa while Casimir took Warsaw.

In 1351 the brothers recognized Casimir III of Poland as suzerain, and in 1355 the king granted him Warsaw. When Casimir III died in 1370, Masovia regained its independence; Siemowit issued a customary law code in 1377 and reformed its coinage. He died in 1381.