|Intro||Youngest daughter of Algirdas, Grand Duke of Lithuania, and his second wife, Uliana of Tver|
|Death||20 April 1434, Płońsk|
Alexandra (Polish: Aleksandra, Lithuanian: Aleksandra; died 20 April 1434 in Płock) was the youngest daughter of Algirdas, Grand Duke of Lithuania, and his second wife, Uliana of Tver. Though Alexandra's exact date of birth is not known, it is thought that she was born in the late 1360s or early 1370s. In 1387, she married Siemowit IV, Duke of Masovia, and bore him thirteen children.
On 12 December 1385, few months after the Union of Krewo, Siemowit IV, Duke of Masovia, reached a compromise with king Jadwiga of Poland and her intended consort king Jogaila (Władysław II Jagiełło), brother of Alexandra. Siemowit IV agreed to cease his rival claims to the Kingdom of Poland, pay homage to Jadwiga and Jogaila, and to assume position of a hereditary vassal to the Polish Crown in exchange for 10,000 Prague groschen and fief Duchy of Belz. The agreement was solidified by marriage of Siemowit IV and Alexandra in 1387.
Alexandra died and was buried in Płock. Her final resting place is likely a church of the Dominican Order.
In her union with Siemowit, Alexandra bore 13 children—five sons and eight daughters.
- Kazimierz II of Masovia
- Trojden II of Masovia
- Władysław I of Płock
- Siemowit V of Masovia
- Alexander of Masovia; who was a diplomat and Bishop of Trento, titular Bishop of Chur, titular Cardinal of Damascus, and Patriarch of Aquileia.
- Euphemia of Masovia; married Bolesław I, Duke of Cieszyn
- Cymburgis of Masovia; married Ernest, Duke of Austria
- Jadwiga of Masovia; married John Garay, son of Nicholas I Garay
- Amelia of Masovia; married William II, Margrave of Meissen
- Anna of Masovia; possibly married Michael Žygimantaitis
- Maria of Masovia; married Bogislaw IX, Duke of Pomerania
- Alexandra of Masovia
- Katarzyna of Masovia; married Michael Žygimantaitis
Grandchildren of Alexandra and Siemowit IV included Frederick III, Holy Roman Emperor, Przemyslaus II, Duke of Cieszyn, Sophie of Pomerania, Duchess of Pomerania and Dorothy Garai.