William III de Ferrers, 5th Earl of Derby (1193 – 28 March 1254) was an English nobleman and head of a family which controlled a large part of Derbyshire including an area known as Duffield Frith.
He was born in Derbyshire, England, the son of William de Ferrers, 4th Earl of Derby and Agnes of Chester, a daughter of Hugh of Kevelioc, Earl of Chester and Bertrada de Montfort. He succeeded to the title in 1247, on the death of his father and, after doing homage to King Henry III, he had livery of Chartley Castle and other lands of his mother's inheritance. He had accompanied King Henry to France in 1230 and sat in parliament in London in the same year.
He had many favours granted to him by the king, among them the right of free warren in Beaurepair (Belper), Makeney, Winleigh (Windley), Holbrooke, Siward (Southwood near Coxbench), Heyhegh (Heage) Cortelegh (Corkley, in the parish of Muggington), Ravensdale, Holland (Hulland), and many other places,
Like his father, he suffered from gout from youth, and always traveled in a litter. He was accidentally thrown from his litter into water, while crossing a bridge, at St Neots, in Huntingdon and although he escaped immediate death, yet he never recovered from the effects of the accident. He died on 28 March 1254, after only seven years, and was succeeded by his son Robert de Ferrers, 6th Earl of Derby.
William de Ferrers is buried at Merevale Abbey, Warwickshire, England. His widow died on 12 March 1280.