Masovian Voivodeship or Mazovia Province (Polish: województwo mazowieckie [vɔjɛˈvutstfɔ mazɔˈvjɛtskʲɛ]), is the largest and most populous of the sixteen Polish provinces, or voivodeships, created in 1999. It occupies 35,579 square kilometres (13,737 sq mi) of east-central Poland, and has 5,324,500 inhabitants. Its principal cities are Warsaw (1.729 million) in the centre of the Warsaw metropolitan area, Radom (226,000) in the south, Płock (127,000) in the west, Siedlce (77,000) in the east, and Ostrołęka (55,000) in the north. The capital of the voivodeship is the national capital, Warsaw.
The province was created on January 1, 1999, out of the former Warsaw, Płock, Ciechanów, Ostrołęka, Siedlce and Radom Voivodeships, pursuant to the Polish local government reforms adopted in 1998. The province's name recalls the traditional name of the region, Mazowsze (sometimes rendered in English as "Masovia"), with which it is roughly coterminous. However, southern part of the voivodeship, with Radom, historically belongs to Małopolska (Lesser Poland), while Łomża and its surroundings, even though historically part of Masovia, now is part of Podlaskie Voivodeship.
Masovian Voivodeship, 1526–1795 (Polish: Województwo Mazowieckie) was an administrative region of the Kingdom of Poland, and of the Polish-Lithuanian Commonwealth, from the 1526 to the partitions of the Polish-Lithuanian Commonwealth (1795). Together with Płock and Rawa Voivodeships, it formed the province of Masovia. Its area was 23,200 km2., divided into ten lands (see ziemia). The seat of the voivode was Warsaw, local sejmiks also convened in Warsaw, at St. Martin's church.
The voivodeship was officially created by King Zygmunt I Stary on December 27, 1529, three years after incorporation of the Duchy of Masovia into the Kingdom of Poland. In the Senate of the Polish-Lithuanian Commonwealth, it had eight senators. These were: the Voivode of Mazovia, the Castellan of Czersk, and Castellans of Wizna, Wyszogrod, Zakroczym, Warszawa, Ciechanów, and Liw.
Zygmunt Gloger in his monumental book Historical Geography of the Lands of Old Poland gives a detailed description of Masovian Voivodeship: