William I de Ferrers, 3rd Earl of Derby (died 31 December 1189) was a 12th-century English Earl who resided in Tutbury Castle in Staffordshire and was head of a family which controlled a large part of Derbyshire known as Duffield Frith. He was also a Knight Templar. William was the son of Robert de Ferrers, 2nd Earl of Derby, and his wife, Margaret Peverel. He succeeded his father as Earl of Derby in 1162. He was married to Sybil, the daughter of William de Braose, 3rd Lord of Bramber, and Bertha of Hereford.