William Hayley (November 9, 1745 - November 12, 1820), was an English writer, best known as the friend and biographer of William Cowper.

Born at Chichester, he was sent to Eton in 1757, and to Trinity Hall, Cambridge, in 1763; his connection with the Middle Temple, London, where he was admitted in 1766, was merely nominal. In 1767 he left Cambridge and went to live in London. Two years later he married Eliza, daughter of Thomas Ball, dean of Chichester. His private means enabled Hayley to live on his patrimonial estate at Eartham, Sussex, and he retired there in 1774.

He had already written many occasional poetical pieces, when in 1771 his tragedy, The Afflicted Father, was rejected by David Garrick. In the same year his translation of Pierre Corneille's Rodogune as The Syrian Queen was also declined by George Colman. Hayley won the fame he enjoyed amongst his contemporaries by his poetical Essays and Epistles; a Poetical Epistle to an Eminent Painter (1780), addressed to his friend George Romney, an Essay on History (1780), in three epistles, addressed to Edward Gibbon; Essay on Epic Poetry (1782) addressed to William Mason; A Philosophical Essay on Old Maids (1785); and the Triumphs of Temper (1781). The last-mentioned work was so popular as to run to twelve or fourteen editions; together with the Triumphs of Music (Chichester, 1804) it was ridiculed by Byron in English Bards and Scotch Reviewers.

So great was Hayley's fame that on Thomas Warton's death in 1790 he was offered the laureateship, which he refused. In 1792, while writing the Life of Milton, Hayley made Cowper's acquaintance. A warm friendship sprang up between the two which lasted till Cowper's death in 1800. Hayley indeed was mainly instrumental in getting Cowper his pension. In 1800 Hayley also lost his natural son, Thomas Alphonso Hayley, to whom he was devotedly attached. He had been a pupil of John Flaxman's, to whom Hayley's Essay on Sculpture (1800) is addressed. Flaxman introduced William Blake to Hayley, and after the latter had moved in 1800 to his marine hermitage at Felpham, Sussex. Blake settled near him for three years to engrave the illustrations for the Life of Cowper. This, Hayleys best known work, was published in 1803-1804 (Chichester) in 5 vols.

In 1805 he published Ballads founded on Anecdotes of Animals (Chichester), with illustrations by Blake, and in 1809 The Life of Romney. For the last twelve years of his life Hayley received an allowance for writing his Memoirs. He died at Felpham on the 12th of November 1820. Hayley's first wife died in 1797; her mind had been seriously affected, and Since 1789 they had been separated. He married in 1809 Mary Welford, but they also separated after three years. He left no children.