William Hay, 18th Earl of Erroll

William George Hay, 18th Earl of Erroll KT GCH PC (21 February 1801 – 19 April 1846), styled Lord Hay between 1815 and 1819, was a Scottish peer and politician.

Erroll was the son of William Hay, 17th Earl of Erroll, and his wife Alice (née Eliot). His paternal grandfather was James Hay, 15th Earl of Erroll, son of William Boyd, 4th Earl of Kilmarnock (who was attainted with his titles forfeited in 1746). He became heir apparent to the earldom in 1815 on the death of his elder brother, Lord Hay. He was educated at Eton.

Erroll succeeded his father in the earldom in 1819, aged 18. In 1823 he was elected a Scottish Representative Peer and took his seat in the House of Lords. He was Master of the Horse to Queen Adelaide from 1830 to 1834. In 1831 he was sworn of the Privy Council and created Baron Kilmarnock, of Kilmarnock in the County of Ayr, in the Peerage of the United Kingdom, a revival of the Kilmarnock title held by his great-grandfather. When the Whigs came to power under Lord Melbourne in 1835, Erroll was appointed Master of the Buckhounds. In 1839 he was promoted to Lord Steward of the Household on the decease of the Duke of Argyll, a post he held until the administration fell in 1841.

Apart from his political career Lord Erroll was also Knight Marischal of Scotland from 1832 to 1846 and Lord-Lieutenant of Aberdeenshire from 1836 to 1846.

Lord Erroll married Lady Elizabeth FitzClarence, the illegitimate daughter of William IV and Dorothy Jordan, on 4 December 1820. They were the parents of four children:

• Lady Ida Harriet Augusta Hay (18 October 1821 – 22 October 1867), one of the Queen's bridesmaids, married Charles Noel, 2nd Earl of Gainsborough.
• William Harry Hay, 19th Earl of Erroll (3 May 1823 – 3 December 1891), married Eliza Amelia Gore on 20 September 1848.
• Lady Agnes Georgiana Elizabeth Hay (12 May 1829 – 18 December 1869), who married James Duff, 5th Earl Fife on 16 March 1846. Their son, Alexander Duff, 1st Duke of Fife, married Princess Louise, daughter of King Edward VII.
• Lady Alice Mary Emily Hay (7 July 1835 – 7 June 1881) married Charles Edward Louis Casimir Stuart (1824–1882), nephew of fraud John Sobieski Stuart.

Lord Erroll died in London in April 1846, aged 45, and was succeeded by his eldest son, William. The Countess of Erroll died in January 1856, aged 54.