William III de Cantelou (died September 25, 1254) was de facto Baron Abergavenny in right of his wife, Eve de Braose, of the de Braose dynasty of Welsh Marcher Lords, the de jure Baroness.

The name has also been transcribed as de Cantilupe - as with Thomas de Cantilupe, his brother, who became Chancellor of England and Bishop of Hereford - de Cantilou and also de Cantelupe. His chief residences were at Calne in Wiltshire and Aston Cantlow in Warwickshire before inheriting the Abergavenny estates.

He was the son of William II de Cantelou and Millicent de Gournay. He married Eve de Braose, daughter of William de Braose, Baron Bergavenny and his wife, Eva Marshal daughter of the famous William Marshall, 1st Earl of Pembroke, before 15 February 1248. They had three children:

1. Milicent de Cantelou, married (1) John de Montalt; (2) Eudo de la Zouche
2. Joan de Cantelou (died by June 1271), married Henry Hastings
3. George de Cantelou (died 1273).

He died "in the flower of his youth" in 1254. Simon de Montfort, a close friend of the family, was the chief mourner at William's funeral. His arms were: De gueules, à trois fleurs de lys d'or.

• Armoriaux online, Rôle d'armes Glover, 27. Guillaume Cauntelo (Will'm de Cantelowe).

• Braose Family Web

• Sir Bernard Burke C.B., LL.D., Ulster King of Arms (compiler). (1996). A Genealogical History of the Dormant, Abeyant, Forfeited and Extinct Peerages of the British Empire. Baltimore:: Genealogical Publishing Co, pg. 101.

• George Edward Cokayne (revised by others later). (2000). The Complete Peerage of England, Scotland, Ireland, Great Britain, and the United Kingdom, Extant, Extinct, or Dormant, I-XIII (in 6). Stroud: Sutton Publishing Limited, II:129.

• Frederick Lewis Weis (with additions and corrections by Walter Lee Sheppard, Jr. and assisted by David Faris). (1992). Ancestral Roots of Certain American Colonists Who Came to America before 1700. Baltimore: Genealogical Publishing Co, Line 66.29. Lines: 39-29, 39A-29, 93A-28, 232A-32