Lord William Pitt Lennox (20 September 1799 – 18 February 1881) was a British Army officer and writer.
Lennox, fourth son of Charles Lennox, 4th Duke of Richmond, and the former Lady Charlotte Gordon, was born at Winestead Hall, Yorkshire, 20 September 1799, and was a godson of William Pitt and a cousin of Charles James Fox. He was educated at Westminster School from 1808 to 1814.
On 13 May 1813, while still at school, he was gazetted to a cornetcy in the Royal Horse Guards, and on 8 August 1814 accompanied the Duke of Wellington as an attaché in his embassy to Paris. In 1815 he was attached to General Sir Peregrine Maitland's staff, was present at his mother's memorable ball in Brussels, and saw some portion of the battle of Waterloo, but was prevented by the effects of a horse accident, which cost him the sight of one eye, from taking an active part in it. For three years after Waterloo he acted as an aide-de-camp to Wellington. He then joined his regiment in England, was promoted to be a captain 28 March 1822, and retired by the sale of his commission 25 March 1829. He was an extra aide-de-camp to his father while he was governor-general of Canada, 1818–1819, and was one of the pages at the coronation of George IV, 19 July 1821.
He represented King's Lynn, Norfolk, in conjunction with Lord George Bentinck, as a moderate reformer and a supporter of the government from 10 December 1832 to 29 December 1834, and spoke on the Reform Bill, on fees paid on vessels in quarantine, and on the Anatomy Bill.