Janusz III of Masovia

Janusz III Mazowiecki (or Janusz III of Masovia) (1502–1526) was a Polish duke of Masovia, and the last male of the Masovian Piast dynasty.

Until 1518 Masovia was under the regency of his mother Anna Radziwiłł, then Janusz shared the regency with his brother Stanisław until Stanisław's death in 1524. After Janusz's own heirless death in 1526 the Duchy of Masovia was annexed into the Kingdom of Poland.