Diana Denyse Hay, 23rd Countess of Erroll (5 January 1926 – 16 May 1978, Oban, Scotland) was a British noblewoman.^[1]

Diana was the only child of Josslyn, Earl of Errol and his first wife Lady Idina Sackville. She was born in Kenya. Their home was a bungalow on the slopes of the Aberdare Range which they called Slains, after the former Hay family seat of Slains Castle which was sold by Hay's grandfather, the 20th Earl, in 1916. The bungalow was sited alongside the high altitude farms which other white Kenyans were establishing at the time.

When her mother divorced her father in 1930, she was taken home to England to be raised firstly by her uncle The Earl De La Warr and then by her aunt, Lady Avice Sackville, in Wiltshire.

When her father was murdered in 1941, she inherited the earldom of Erroll and the lordship of Hay, while the barony of Kilmarnock, which could only be inherited by a male heir, passed to her uncle Gilbert Boyd. She also inherited the hereditary position Lord High Constable of Scotland.^[2]

After the passing of the Peerage Act 1963 which allowed suo jure peeresses to take a seat in the House of Lords, Lady Erroll did so, with eleven other peeresses.

On 19 December 1946, at St Margaret's, Westminster, Lady Erroll married Sir Iain Moncreiffe of that Ilk, the Chief of Clan Moncreiffe and a herald at the Court of the Lord Lyon.^[1] Together, they were the parents of three children:^[3]

• Merlin Sereld Victor Gilbert, 24th Earl of Erroll (b. 1948), who married Belgian born Isabelle Jacqueline Laline Astell Hohler, in 1982.^[3]
• Hon. Peregrine David Euan Malcolm Moncreiffe of that Ilk (b. 1951), who became Chief of Clan Moncreiffe and who married Miranda Mary Fox-Pitt, in 1988.^[3]
• Lady Alexandra Victoria Caroline Anne Moncreiffe Hay (b. 1955), who married Jolyon Connell.^[3]