William Hay, 17th Earl of Erroll (12 March 1772 – 26 January 1819), known as Lord Hay until 1778, was a Scottish peer.^[1]

Erroll was the son of James Hay, 15th Earl of Erroll and his second wife, Isabella Carr, the eldest daughter and co-heiress of William Carr of Etal in Northumberland. His elder sister, Lady Augusta Hay (the first wife George Boyle, 4th Earl of Glasgow), succeeded to the Etal estate in 1806. From his father's first marriage to Rebecca Lockhart, he had one half-sibling, Lady Mary Hay, who married Gen. John Scott of Balcomie.^[2]

His paternal grandparents were William Boyd, 4th Earl of Kilmarnock and Lady Anne Livingston (a daughter of James Livingston, 5th Earl of Linlithgow).^[2][3]

In 1798, he succeeded to the earldom of Erroll following the death of his elder brother George, who died without issue.^[2]

In 1805, he was appointed Knight Marischal of Scotland, serving until 1809.^[4] He also served as a Scottish representative peer (Whig) from 1806 to 1807 and, again, from 1818 to 1819.^[2]

From 1817 to 1819, he was Lord High Commissioner to the General Assembly of the Church of Scotland.^[2]

He firstly married Jane Bell (d. 1793), the second daughter of Matthew Bell of Wolsingham.^[5] in 1792. Before her death in 1793, they had one child together:^[2]

• Lady Dulcibella Hay (1793–1885), who married The Ven. Charles Nourse Wodehouse, Archdeacon of Norwich, second son of the Rev. Philip Wodehouse, Prebend of Norwich (brother of John Wodehouse, 1st Baron Wodehouse, and second son of Sir Armine Wodehouse, 5th Baronet) and Apollonia Nourse (daughter and co-heiress of John Nourse), in 1821.^[2]

Secondly, he married Alicia Eliot (d. 1812), the third daughter of Samuel Eliot, Esq. of Antigua, and Alice (née Byam) Eliot (a daughter of Col. William Byam of Antigua), in 1796. Alicia was the sister-in-law of Sir Thomas Stapleton, 6th Bt. (also the 16th Baron Le Despencer). Together, they were the parents of seven children, including:^[2]

• James Hay, Lord Hay (1797–1815), who was killed at the Battle of Quatre Bras.^[2]
• Lady Alicia Hay (1798–1799), who died young.^[2]
• Lady Isabella Hay (1800–1868), who married Lt.-Gen. William Wemyss, second son of Lt.-Gen. William Wemyss MP and Frances Erskine (a daughter of Sir William Erskine, 1st Baronet) in 1820.^[2]
• William Hay, 18th Earl of Erroll (1801–1846), who married Lady Elizabeth FitzClarence, an illegitimate daughter of King William IV by his mistress Dora Bland ("Mrs Jordan").^[2]
• Lady Harriet Jemima Hay (1803–1837), who married Daniel Gurney, the High Sheriff of Norfolk, in 1822.^[2]
• Lady Caroline Hay (1805–1877), who married John Morant of Brockenhurst in 1823.^[2]
• Hon. Samuel Hay (1807–1847), who married Louisa Pleydell-Bouverie, the only daughter of Vice-Admiral Hon. Duncombe Pleydell-Bouverie (second son of Jacob Pleydell-Bouverie, 2nd Earl of Radnor) and Louisa May (second daughter of Joseph May of Hale House), in 1832.^[2]
• Lady Emma Hay (1809–1841), who married Rear-Admiral James Erskine Wemyss, eldest son of Lt.-Gen. William Wemyss, in 1826.^[2]

In 1816, he married, thirdly, to Harriet Somerville (d. 1864), sister of Mark Somerville, 16th Lord Somerville and third daughter of Lt.-Col. Hon. Hugh Somerville (second son of James Somerville, 13th Lord Somerville), by his second wife, Mary Digby (eldest daughter of Hon. Wriothesley Digby, son of William Digby, 5th Baron Digby). They were the parents of three children, including:^[2]

• Rev. Hon. Somerville Hay (1817–1853), who married Lady Alicia Diana Erskine, third daughter of Henry Erskine, 12th Earl of Buchan, in 1843.^[2]
• Lady Fanny Hay (1818–1853), who married the Rev. Stephen Ralph Cartwright, Rector of Aynhoe, in 1848.^[2]
• Lady Margaret Hay (1819–1891), who married Frederick Astell Lushington, sixth son of Sir Henry Lushington, 2nd Baronet and Fanny Maria Lewis (sister and co-heiress of Matthew Gregory Lewis) in 1846.^[2]

Lord Erroll died on 26 January 1819 and was succeeded by his eldest surviving son, William. His widow, the Dowager Countess of Erroll, died in 1864.^[2]