William I (Willem Frederik, Prince of Orange-Nassau; 24 August 1772 – 12 December 1843) was a Prince of Orange, the King of the Netherlands and Grand Duke of Luxembourg.

He was the son of the last Stadtholder of the Dutch Republic, who went into exile to London in 1795 because of the Batavian Revolution. As compensation for the loss of all his father's possessions in the Low Countries, an agreement was concluded between France and Prussia in which William was appointed ruler of the newly created Principality of Nassau-Orange-Fulda in 1803; this was however short-lived and in 1806 he was deposed by Napoleon. With the death of his father in 1806, he became Prince of Orange and ruler of the Principality of Orange-Nassau, which he also lost the same year after the dissolution of the Holy Roman Empire and subsequent creation of the Confederation of the Rhine at the behest of Napoleon. In 1813, when Napoleon was defeated at the Battle of Leipzig, the Orange-Nassau territories were returned to William and he was asked as well to become the Sovereign Prince of the United Netherlands. He proclaimed himself King of the Netherlands on 16 March 1815. In that year, William I concluded a treaty with King Frederick William III in which he ceded the Principality of Orange-Nassau to Prussia in exchange for becoming the new Grand Duke of Luxembourg. In 1839, he furthermore became the Duke of Limburg as a result of the Treaty of London. After his abdication in 1840, he styled himself King William Frederick, Count of Nassau.