Janusz I of Warsaw (pl: "Janusz I warszawski"), also known as Janusz I the Old (pl: "Janusz I Starszy") (c. 1347/52 – 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).