The Privilege of Mielnik was an act promulgated on October 25, 1501, at Mielnik by Poland's King Alexander Jagiellon. It substantially expanded the powers of the Senate and the magnates, at the expense of the king. Furthermore, it de facto introduced a republican form of government. However, the privilege was not confirmed by the king after his election, and in 1504 was rejected by the Sejm (see Łaski's Statutes).
After the death of Casimir IV Jagiellon (1492), the personal union between the Kingdom of Poland and the Grand Duchy of Lithuania was broken. The new king of Poland was John I Albert, while Lithuania was governed by his brother, Alexander. Following his father, Olbracht tried to win the support of the nobility, granting it several concessions in the Statutes of Piotrków. The failed invasion of Moldavia in 1497 complicated Poland's internal situation, and strengthened the position of the magnates. At the same time, Lithuania was attacked by Ivan III of Russia, and in 1499 both nations signed the Union of Kraków and Vilnius, which revived the Polish-Lithuanian alliance. In 1500 John Albert died, and on October 3, 1501, Alexander became the King of Poland. His election worsened the Lithuanians' position, and they agreed to the Union of Mielnik. Alexander himself had to sign the Privilege of Mielnik, which limited his authority and expanded the powers of the Senate.
The Privilege was as follows:
The new political system of Poland, created by the privilege, was disliked by the nobility, which refused to pay taxes and to take part in the pospolite ruszenie. Starostas did not cooperate with senators, and during the 1504 Sejm, which took place in Piotrków Trybunalski, Jan Laski initiated attack on the powerful Lesser Poland magnates. As a result, the nobility managed to regain their position, and during the 1505 Sejm in Radom, the Nihil novi act was introduced, which voided the Privilege of Mielnik.