|Birth||17 May 1500|
|Death||8 August 1524|
Stanisław of Masovia (pl: Stanisław mazowiecki; 17 May 1501 – 8 August 1524), 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.
He was the eldest 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, Stanisław and his younger brother Janusz III 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, the young princes received their whole patrimony as a fief.
Stanisław 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.
In 1519, fulfilling his duties as Polish vassals, Stanisław and Janusz III intervened in the Polish-Teutonic War, sending auxiliary troops to the Polish King, and in the winter of 1519-1520 they personally captured several town in Masuria. At the same time, Stanisław secretly talks with the Teutonic Knights for a ceasefire, which finally took place in December 1520, a few months before a peace treaty ended for good the war between Poland and the Teutonic Order.
In their private lives, both Stanisław and his brother are heavily inclined to drink and women; however, in order to continue his bloodline, in 1523 Stanisław started negotiations for marriage with Princess Hedwig of Poland, only surviving daughter of King Sigismund I and his first wife, Barbara Zápolya. The wedding never took place; one year later, and likely for his dissolute lifestyle, Stanisław died on 8 August 1524, probably shortly before Noon. He was buried at St. John's Archcathedral, Warsaw.
The sudden deaths of Stanisław, and two years later of his younger brother Janusz III, caused suspiciones in their time. The main suspect was a Płock lady called Katarzyna Radziejowska, who after being seduced and abandoned by both princes, in revenge she could poisoned firstly Anna Radziwiłł, then Stanisław and finally Janusz III. Declared guilty, she and her supposed accomplice where tied naked to poles and beaten for hours, and finally burned alive. The hurry where the sentence was carried caused even more suspicions that in fact the real instigator of the crimes was Queen Bona. The controversy was so intense that King Sigismund I, in order to clarify the matter once and for all, ordered an investigatation, as a result of which was declared a special edict on 9 February 1528 who ruled that the princes "weren't victims of a human hand, but was the will of the Almighty Lord that caused their deaths".
According to Jan Długosz, the real cause of the death of both princes could be an inherited disease of the Masovian princes: tuberculosis.