Masovia , Pol. Mazowsze, historic region, almost coextensive with Warsaw province, central Poland. At the death (1138) of Boleslaus III, Masovia became an independent duchy under the Piast dynasty. It became a suzerainty of Great Poland in 1351 and was finally united with it in 1526. Masovia passed to Prussia during the 18th-century partitions of Poland and was later a part of the Russian Empire. It reverted to Poland in 1918.