Duke William of Austria, known as the Ambitious, Duke of Inner Austria (c. 1370 – July 15, 1406 in the same place), was a member and head of the Leopoldinian Line, ruler of Carinthia, Styria and Carniola.

Born in Vienna, he was the oldest son of Duke Leopold III and his wife Viridis Visconti and ruled in Carinthia, Styria and Carniola.

His engagement with Jadwiga of Hungary, youngest daughter of the neighboring king, 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!" William's interests lay in having Hungarian lands, promised by Jadwiga's father King Louis I of Hungary, as dowry for Jadwiga, acquiring power in neighboring Hungary, and possibly even being elevated to kingship of Hungary as Louis had two kingdoms, i.e, also that of Poland. Hungarian magnates were famous for electing kings and not directly allowing them to inherit.

Fate reversed part of these plans, however even to a grander prospect: Jadwiga was chosen as reigning queen of Poland, a country far north of Austria, and was confirmed in that position in 1384. William was repudiated, because there were more fitting marriage prospects from the Polish point of view. After the engagement was dissolved, William married her cousin Joan II of Naples from the House of Anjou (1373-1435) as her first husband, before her reign. However, the marriage did not produce any offspring. By contrast, William's younger brother 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.

In 1394, after death of his first cousin, Duke Albert IV of Lower Austria, he tried to obtain control over the lands of the Albertinian Line's territories during the minority of Albert V. However, he never achieved that.

William died in 1406 in Vienna. He is buried in the Dukes' Catacomb in Vienna's Cathedral of Saint Stephan.

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