Harriet Arbuthnot (10 September 1793 – 2 August 1834) was an early 19th-century English diarist, social observer and political hostess on behalf of the Tory party. During the 1820s she was the closest woman friend of the hero of Waterloo and British Prime Minister, the 1st Duke of Wellington. She maintained a long correspondence and association with the Duke, all of which she recorded in her diaries, which are consequently extensively used in all authoritative biographies of the Duke of Wellington.