Duchy of Masovia was a district principality and a fiefdom of the Kingdom of Poland, existing during the Middle Ages. The state was centered in Mazovia in the northeastern Kingdom of Poland, and during its existence, its capital was located in the Płock, Czersk and Warsaw. It was formed in 1138 from the territories of the Kingdom of Poland, following its fragmentation, that was started by the testament of Bolesław III Wrymouth. The country existed in the years: 1138–1275, 1294–1310, 1370–1381, and 1495–1526, between that time, going through fragmentations of its territory into smaller duchies and its unification. The states formed during its fragmentation were duchies of Kuyavia, Dobrzyń, Czersk, Płock, Warsaw, Rawa and Belz. In 1526, the country was incorporated into the Kingdom of Poland.

Mazovia or Masovia is a historical region in mid-north-eastern Poland. It spans the North European Plain, roughly between Łódź and Białystok, with Warsaw being the unofficial capital and largest city. Throughout the centuries, Mazovia developed a separate sub-culture featuring diverse folk songs, architecture, dress and traditions different from those of other Poles.