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 "Mazovia"), with which it is roughly coterminous. However, southern part of the voivodeship, with Radom, historically belongs to Lesser Poland, while Łomża and its surroundings, even though historically part of Mazovia, now is part of Podlaskie Voivodeship.