- Born: Abt 1055
- Died: 22 Feb 1071, Battle of Cassel, Cassel, Nord-Pas-de-Calais, France about age 16
Arnulf III (c. 1055 - February 22, 1071, in battle), was Count of Flanders and Count of Hainaut, as Arnulf I from 1070 to his death.
He was the eldest son of Baldwin VI, Count of Flanders and Richilde, Countess of Mons and Hainaut. He succeeded his father in both counties. With his untimely death at the Battle of Cassel, Flanders and Hainaut were separated again.