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'Łomża Department' (Polish: ''Departament Łomzyński'') was an administrative division and local government in the Polish Duchy of Warsaw in the years 1806-1815. Its capital city was Łomża and consisted of 10 counties.
From January to July of 1807 the Department was known as ''Białystok Department'' (Departament Białostocki) with the capital of Białystok, but after Treaties of Tilsit, Russia agreed for the creation of the Duchy of Warsaw, but in exchange it was ceded for powiats: Białostocki, Bielski, Sokólski and Drohicki. Thus the capital of the Department had to be moved, and its name changed.
After 1815 most of its lands became a part of the Augustów Voivodeship. In 1867 it was reconstituted as the Łomża Governorate.