William I Frederick, born Willem Frederik Prins van Oranje-Nassau (24 August 1772 – 12 December 1843), is a Prince of Orange and the first King of the Netherlands and Grand Duke of Luxembourg.

In Germany, he is ruler (as Fürst) 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 till 1815.

In 1813, he is named 'Sovereign Prince' of the Netherlands, and proclaimshimself King of the Netherlands and Duke of Luxembourg on March 16, 1815.

In the same year, on 9 June 9,William I becomes also the Grand Duke of Luxembourg and after 1839 he is furthermore the Duke of Limburg.

After his abdication in 1840, he names himself King William Frederick, Count of Nassau.