William Harness (1790–1869) was an English cleric and man of letters.
Born near Wickham in Hampshire on 14 March 1790, he was son of John Harness, M.D., commissioner of transports, and elder brother Henry Drury Harness. In 1796 he went to Lisbon with his father, and in 1802 entered Harrow School. There he knew Lord Byron, with whom he had a physical disability of the leg in common. He went on to Christ's College, Cambridge, where he graduated B.A. 1812, M.A. 1816.
Some letters from Byron to Harness survive from two short periods between February and June 1808, and then in December 1811. All Byron's letters seek to re-establish a recalled Harrow friendship and latterly became increasingly intimate in tone. Harness spent part of Christmas 1811 with Byron and another friend, Francis Hodgson, at Newstead Abbey but the friendship cooled so much immediately after, that there is no correspondence at all. The following month, Byron declared to his friend John Cam Hobhouse that 'Hodgson was with me...& a Mr Harness of Harrow a mighty friend of mine, but I am sick of Harrow things'. Either Harness fended off a Byron advance, or the poet tired of the rather priggish youth.
Harness took holy orders, and was ordained curate of Kilmeston, Hampshire, in 1812. He was curate of Dorking 1814-16, and then preacher at Trinity Chapel, Conduit Street, London, and minister and evening lecturer at St. Anne's, Soho. At Hampstead he was curate from 1823 to 1826, and then, owing to his popularity as a preacher, became incumbent of Regent Square Chapel, St. Pancras, London, from 1826 to 1844. On the opposite side of Regent Square, Edward Irving's chapel was situated, and in 1831 Harness preached a pointed sermon entitled Modern Claims to Miraculous Gifts of the Spirit. On visiting Stratford-on-Avon, and finding the inscription on Shakespeare's monument in a poor state, he had it restored.