William Upcott (1779–1845) was an English librarian and antiquary.
Born in Oxfordshire, he was the illegitimate son of Ozias Humphry by Delly Wickens, daughter of an Oxford shopkeeper, called Upcott from the maiden name of Humphry's mother. His father bequeathed to him his miniatures, pictures, drawings, and engravings, as well as correspondence with many leading figures.
Upcott was initially a bookseller, at first an assistant of Robert Harding Evans of Pall Mall, and then of John Wright of Piccadilly. While at Wright's shop he attracted the attention of John Ireland, William Gifford, and the writers of the Anti-Jacobin who met there, and he witnessed the scuffle there between Gifford and John Wolcot, helping to eject Wolcot.
When Richard Porson was made librarian of the London Institution, Upcott was appointed as his assistant (23 April 1806), and he continued in the same position under William Maltby. On 30 May 1834 he resigned his office.
Upcott spent the rest of his days at 102 Upper Street, Islington. The house in his time was called ‘Autograph Cottage’; among many autographs collected by Upcott was that of William Blake, in 1826, with whom he was on good terms, as his father had been, and made some significant introductions to Blake (Henry Crabb Robinson and Dawson Turner). In imitation of the plan adopted by William Oldys, he fitted up a room with shelves and a hundred receptacles into which he dropped cuttings on different subjects. The Guildhall Library originated in a suggestion by him, and in 1828 he superintended the arrangement of the books in it.