William (4 July 1535 – 20 August 1592), called William the Younger (German: Wilhelm der Jüngere), was Duke of Brunswick-Lüneburg and Prince of Lüneburg from 1559 until his death. Until 1569 he ruled together with his brother, Henry of Dannenberg. William was the son of Ernest I, Duke of Brunswick-Lüneburg.
After several early divisions, Brunswick-Lüneburg re-unified under Duke Magnus II (d. 1373). Following his death, his three sons jointly ruled the Duchy. After the murder of their brother Frederick I, Duke of Brunswick-Lüneburg , brothers Bernard and Henry redivided the land, Henry receiving the territory of Wolfenbüttel.