Old St. Paul's, also titled Old Saint Paul's: A Tale of the Plague and the Fire, is a novel by William Harrison Ainsworth serially published in 1841. It is a historical romance that describes the events of the Great Plague of London and the Great Fire of London. It was the basis for the 1914 silent film Old St. Paul's.

William Harrison Ainsworth was an English historical novelist born at King Street in Manchester. He trained as a lawyer, but the legal profession held no attraction for him. While completing his legal studies in London he met the publisher John Ebers, at that time manager of the King's Theatre, Haymarket. Ebers introduced Ainsworth to literary and dramatic circles, and to his daughter, who became Ainsworth's wife.