Victor Alexander Sereld Hay, 21st Earl of Erroll and 4th Baron Kilmarnock, KCMG (17 October 1876 – 20 February 1928), was a British diplomat, a writer and briefly a member of the House of Lords.
Erroll was the first son of Charles Hay, 20th Earl of Erroll (1852–1927) and his wife Mary, daughter of Edmund and Lady Harriett L'Estrange. He succeeded his father in the earldom in 1927. In 1900, he married Mary Lucy Victoria, only daughter of Sir Allan Mackenzie, 2nd Baronet, of Glen Muick, Aberdeenshire, and they had two sons and one daughter.
He held the earldom only briefly, and was succeeded by his elder son, Lord Kilmarnock, in 1928.
The young Hay entered the diplomatic service and was promoted Attaché, 1900, Third Secretary, 1902, Second Secretary, 1906, First Secretary, 1913. He was First Secretary in Copenhagen, 1918–19, then briefly Chargé d'Affaires in Berlin, on the United Kingdom's resumption of diplomatic relations with Germany in 1920, until the arrival of a British Ambassador. He remained in Berlin as Counsellor until November, 1921, and served finally as British High Commissioner to the Inter-Allied Rhineland High Commission, from December 1920 to May 1928.
He was also an author and belonged to the St James's Club.