Janusz III of Masovia

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.

After the death of their father on 28 October 1503, Janusz III and his younger brother Stanisław inherited his domains, but because they are minors, remained under the regency of their mother.

Most of the Masovian inheritance (except Czersk, who was already given to Konrad III as hereditary fief in 1495) was seriously threatened by the Kingdom of Poland at the time of Konrad III's death, and wasn't secured in his sons' hands until 14 March 1504, when by a ruling of King Alexander (who feared the protest of the local nobility) the young princes received their whole patrimony as a fief.

Janusz III and his brother took the government in 1518, due to the constant riots of the local nobility. Despite this, Anna Radziwiłł retained the real power in Masovia until her death in 1522. In the same year when they attained their majority, both princes attended the wedding ceremony of King Sigismund I the Old with Bona Sforza in Kraków.

As Polish vassals, during 1519-1520 Janusz III and his brother participated in the Polish-Teutonic War sending auxiliary troops to the Polish King.

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