William III (1224 – 6 June 1251) was the lord of Dampierre from 1231 and count of Flanders from 1247 until his death. He was the son of William II of Dampierre and Margaret II of Flanders. Margaret inherited Flanders and Hainault in 1244 and immediately the War of the Succession of Flanders and Hainault began between William and his brothers, the Dampierre claimants, and the children of Margaret's first marriage to Bouchard of Avesnes. Margaret favoured William and declared him her heir. In 1246, Louis IX of France intervened to arbitrate the conflict and declared Flanders to William and Hainault to John I of Avesnes. Margaret officially invested William as count in 1247.