Janusz III of Masovia

Janusz III Mazowiecki (or Janusz III of Masovia) (1502–1526) was a Polish duke of Masovia, last male of the Masovian Piast dynasty. Son of Konrad III Rudy. Until 1518 Masovia was under regency of his mother Anna Radziwill, then he was a co-regent with his brother Stanislaw until Stanislaw's death in 1524. After his heirless death in 1526, the Duchy of Masovia was annexed into the Kingdom of Poland.

Janusz was the younger son of Konrad III Rudy and Anna Radziwill, born in winter 1502. After his father's death in 1503 Janusz and his brother Stanislaw inhertited the Czersk Land, and with support of Alexander Jagiellon, the throne of Duchy of Masovia, but due to their young age the throne was taken by their mother, Anna, who became the regent.
In 1518, both brothers were invited to the wedding of Sigismund I the Old and Bona Sforza. Janusz and Stanislaw fought on the Polish side during the Polish-Teutonic War (1519–1521).
Anna reigned until 1518, past the time both brothers reached the age in which they could became rulers. Eventually the disapproval of local nobility resulted in her officially transferring the power to her sons, but in fact she continued to rule the Duchy until 1522.
After their mother's death, both brothers quickly became known for their love of "women and wine". Both brothers acted as co-rulers until Stanislaw's death in 1524. However Janusz ruled only until his death in the night of 9 to 10 March, 1526. He was buried in St. John's Cathedral, Warsaw. With his death, the male line of Masovian Piasts, originating with Siemowit III of Masovia died out.
The death of both brothers caused unrest, as accusations that they were murdered became widespread. Eventually, King Sigismund himself looked into the matter, and concluded that there was no foul play. A contemporary historian, Marcin Bielski, suggested that both brothers died due to alcohol poisoning; others suggested tuberculosis.
Soon after his death, the Duchy of Masovia was annexed into the kingdom of Poland, despite resistance from some of the Masovian nobility. They tried to retain their independence and argued that the Duchy should be inherited by the female relatives (such as Anna or Sophia of Masovia). The Polish king refused to recognize their demands, and stood by the agreements that made him the heir to the Duchy, reuniting it with Poland. The Duchy, which would become a significant asset of the Polish Jagiellon dynasty, would retain some autonomy till 1576.