Casimir I of Warsaw (pl: Kazimierz I warszawski; 1329/31 – 26 November/5 December 1355[1]), was a Polish prince, member of the House of Piast, Duke of Czersk 1341–1349 jointly with his brother, after 1345 ruler over Rawa Mazowiecka jointly with his brother, after 1349 sole ruler over Warsaw, after 1351 sole ruler over Sochaczew, Polish vassal since 1351.
He was the third and youngest son of Trojden I of Czersk and Maria, daughter of Yuri I, King of Halych-Volhynia.