Janusz I (c. 1347-1352[1] – 1429) was Duke of Warsaw (from 1373), one of the dukes of Masovia after its division amongst the sons of Duke Siemowit III.
Among the lands in his domain were the areas of Warsaw, Nur, Łomża, Ciechanów and Różan. In 1374 he inherited Zakroczym and in 1381 the lands of Czersk, Liw and Wyszogród. In 1376 he married princess Danutė (Danuta Anna), daughter of Lithuanian Duke Kęstutis. Because of his family ties, he served as a mediator between king of Poland Jogaila and the Grand Duke of Lithuania Vytautas.
In 1386 Janusz became a hereditary vassal of Poland. In 1391 he inherited the title of the Duke of Podlaskie. In 1406 he moved the capital of Masovia from Czersk to Warsaw. In 1409 he organised a banner of cavalrymen to aid his liege, the king of Poland, in the war against the Teutonic Order. The following year together with his banner, despite his age, he took part in the Battle of Grunwald.
With Danuta, he fathered three sons: Janusz Młodszy (Janusz the Younger), Bolesław Januszowic and Konrad Januszowic. He outlived them all, and passed his territories to his grandson, Bolesław IV of Warsaw.