Isabel de Warenne, 4th Countess of Surrey (died 12 July 1203) was an English peeress. She was the only surviving heir of William de Warenne, 3rd Earl of Surrey and his wife, Adela, the daughter of William III of Ponthieu.

In 1148, de Warenne inherited her father's lands and the earldom of Surrey and was married to William of Blois, the younger son of King Stephen, that year. The marriage occurred at a critical moment in The Anarchy as part of the king's attempt to control the de Warenne lands. The couple did not have any children and after William's death in 1159, William X, Count of Poitou sought her hand in 1162/3, but Thomas Becket refused a dispensation from affinity on the grounds of consanguinity.

In April 1164, the countess married Hamelin Plantagenet, the half-brother of King Henry II, who became jure uxoris Earl of Surrey. They had four surviving children:

• William, later 6th Earl of Surrey (1166–1240)
• Adela (born c. 1170, date of death unknown), married Robert of Naburn and William FitzWilliam and was also a mistress of King John.
• Isabel (died 30 November 1234), married Robert de Lacy and Gilbert de l'Aigle, Lord of Pevensey.
• Matilda, married Henry, Count of Eu and Henry de Stuteville.

Hamelin died in 1202 and the countess a year later. She was buried alongside him in the chapter house of Lewes Priory.