William of Austria, known as the Ambitious (born around 1370 in Vienna; died July 15, 1406 in the same place), was a Duke of Austria, and as a member of the Leopoldinian Line, regent of Carinthia, Styria and Carniola.

He was the oldest son of Duke Leopold III (1351-1386) and his wife Viridis , née Visconti, Princess of Milan (1350-1414) and ruled in Carinthia, Styria and Carniola. His engagement with Hedwig of Poland was one of the first attempts of the House of Habsburg to extend their sphere of influence in Eastern Central Europe under the maxim of "Tu felix Austria, nube!", meaning "...you, happy Austria, marry!" When the engagement was dissolved, William married her cousin Joan of Naples from the House of Anjou (1373-1435). However, the marriage did not produce any offspring. By contrast, William's younger brohter Ernest the Iron, Duke from 1377 to 1424, through his marriage with a Piast princess from Warsaw, Cymburgis of Masovia, became the ancestor of all Habsburgs who later were German and Austrian Emperors from 1440 to 1918.

Duke William is buried in the Dukes' Catacomb in Vienna's Cathedral of Saint Stephan.