Janusz III of Masovia (Polish: Janusz III mazowiecki; ca. 27 September 1502 – 9/10 March 1526) was a Polish prince and a member of the House of Piast in the Masovian branch. He served as Duke of Czersk, Warsaw, Liw, Zakroczym, and Nur from 1503 to 1524 (under regency until 1518) jointly with his brother Stanisław, and as sole ruler from 1524 until his death in 1526. He was the last male member of the Masovian Piast dynasty.
Family and Early Life
Janusz III was the second son of Konrad III the Red, Duke of Masovia, and his third wife Anna Radziwiłł, daughter of Mikołaj Radziwiłł the Old, Voivod of Vilnius and Grand Chancellor of Lithuania. He was born in the winter of 1502.
Reign
Following the death of their father on 28 October 1503, Janusz III and his older brother Stanisław inherited his domains. Because they were minors, their mother Anna Radziwiłł served as regent. The Masovian inheritance was threatened by the Kingdom of Poland, but on 14 March 1504, King Alexander I Jagiellon confirmed the young princes' possession of their patrimony as a fief.
Janusz III and his brother assumed direct government in 1518, though their mother retained real power until her death in 1522. In the same year, both princes attended the wedding of King Sigismund I the Old to Bona Sforza in Kraków. As Polish vassals, they participated in the Polish–Teutonic War of 1519–1521, sending auxiliary troops to the Polish king.
After Stanisław's death on 8 August 1524, Janusz III became sole ruler. In 1525, he forbade Lutheranism in his domains under penalty of confiscation of property and death.
Death and Legacy
Janusz III died during the night of 9–10 March 1526, at approximately 23 years of age. Contemporary accounts attribute his early death to his dissolute lifestyle, including heavy drinking. Historian Jan Długosz suggested an inherited disease such as tuberculosis, while Marcin Bielski proposed alcohol poisoning. He was buried at St. John's Archcathedral in Warsaw. Rumors of murder circulated, but King Sigismund I investigated and concluded there was no foul play.
With Janusz III's death, the male line of the Masovian Piasts became extinct. The Duchy of Masovia was subsequently incorporated into the Kingdom of Poland, despite resistance from Masovian nobility who sought to preserve autonomy or have the duchy inherited by female relatives. The duchy retained some autonomy until 1576.
Janusz III is depicted in Jan Matejko's famous painting Prussian Homage. Based on analysis of his remains, he belonged to Y-chromosome haplogroup R1b.
