William I (Willem Frederik, Prince of Orange-Nassau) was a Prince of Orange and the first King of the Netherlands and Grand Duke of Luxembourg.
He was the ruler of the Principality of Nassau-Orange-Fulda from 1803 until 1806 and of the Principality of Orange-Nassau in the year 1806 and from 1813 until 1815. In 1813 he proclaimed himself Sovereign Prince of the United Netherlands. He proclaimed himself King of the Netherlands and Duke of Luxembourg on 16 March 1815. In the same year on 9 June William I became also the Grand Duke of Luxembourg and after 1839 he was furthermore the Duke of Limburg. After his abdication in 1840 he styled himself King William Frederick, Count of Nassau.