William III (Dutch: Willem Alexander Paul Frederik Lodewijk; English: William Alexander Paul Frederick Louis; 19 February 1817 – 23 November 1890) was King of the Netherlands and Grand Duke of Luxembourg from 1849 until his death in 1890. He was also the Duke of Limburg from 1849 until the abolition of the duchy in 1866.

William was the son of King William II and Anna Pavlovna of Russia. On the abdication of his grandfather William I in 1840, he became the Prince of Orange. On the death of his father in 1849, he succeeded as king of the Netherlands.

William married his cousin Sophie of Württemberg in 1839 and they had three sons, William, Maurice, and Alexander, all of whom predeceased him. After Sophie's death in 1877 he married Emma of Waldeck and Pyrmont in 1879 and they had one daughter Wilhelmina, who succeeded William to the Dutch throne. Meanwhile, being the last agnatic dynastic descendant of Otto I, Count of Nassau, the throne of the Grand Duchy of Luxembourg passed to his patrilineal seventeenth cousin once removed (and matrilineal third cousin), Adolphe. As of 2022, he is the last Dutch monarch to die whilst on the throne as all his successors have abdicated in favor of their children.