William of Winchester, Lord of Lunenburg

William (11 April 1184, Winchester – 13 December 1213), called William of Winchester, William Longsword, or William of Lüneburg, was the youngest son of Duke Henry the Lion.

William was born in England during his father's exile; he remained there when Henry returned to Saxony and was raised at Richard Lionheart's court. When Henry died in 1195, William inherited Henry's properties around Lüneburg, near Lauenburg, and in the eastern Harz.

In 1202, William married Helen, daughter of King Valdemar I of Denmark. Their only child was Otto (1204-1252), who inherited his father's property.