Janusz I of Warsaw (pl: Janusz I warszawski), also known as Janusz I the Old (pl: Janusz I Starszy) (c. 1347/52[1] – 8 December 1429), was a Polish prince member of the House of Piast in the Masovian branch, from 1373/74 Duke of Warsaw and after the division of the paternal inheritance between him and his brother in 1381, ruler over Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym. In addition, he was a vassal of the Polish Kingdom since 1391 for the fief of Podlachia (only during his lifetime).
He was the eldest son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava. Due to an error of chronicler Jan Długosz was previously assumed that Janusz I was born c. 1329, and it wasn't until modern time that this date could be corrected until a much later one, c. 1346. Evidence of this fact was that only in 1373/74 he received his own duchy (with its capital in Warsaw).
As the result of the partition of Masovia between him and his younger brother Siemowit IV after the death of their father on 16 June 1381, Janusz I finally obtain the totally of his domains: Warsaw, Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym.
- ^ Kazimierz Jasiński: Rodowód Piastów mazowieckich. Poznań - Wrocław 1998, p. 79.