Mazovia (məzōˈvēə) [key] or Masovia məsōˈ– Pol. Mazowsze, historic region, central Poland. At the death (1138) of Boleslaus III, Mazovia 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. Mazovia 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.