William Hay, 4th Earl of Kinnoull (died 28 March 1677) was a Scottish peer and soldier, loyal to King Charles I.

Montrose's nephew, Archibald Napier, 2nd Lord Napier, wrote his uncle from Brussels, 14 June 1648, "At my parting from France there went in my company above fifty men that did belong to my Lord Montrose; amongst which was Monsieur Hay, Kinnoull's brother, and severall others of good quality.

[3] Charles Gordon, 1st Earl of Aboyne writes that Kinnoull accompanied Montrose after his defeat at the Battle of Carbisdale in April 1650, when he was ultimately captured.

[5] Upon the death of his cousin James Hay, 2nd Earl of Carlisle in 1660, Kinnoull succeeded him in the proprietorship of Barbados, but sold it to the Crown in 1661 for a pension.

They had two sons, both of whom succeeded in the earldom:[5] He died 28 March 1677 and was buried in May in Waltham Abbey Church, Essex.