Janusz III of Masovia

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Janusz III of Masovia (pl: Janusz III mazowiecki; ca. 27 September 1502 – 9/10 March 1526), was a Polish prince member of the House of Piast in the Masovian branch. He was a Duke of Czersk, Warsaw, Liw, Zakroczym and Nur during 1503-1524 (under regency until 1518) jointly with his brother, and sole ruler during 1524-1526 as the last male member of the Masovian Piasts.

He was the second son of Konrad III the Red and his third wife Anna, a daughter of Mikolaj Radziwiłł the Old, Voivod of Vilnius and the first Grand Chancellor of Lithuania.