Guglielmo Manzi (25 August 1784 - 21 February 1821) was an Italian classicist and librarian of what was then the Biblioteca Barberiniana.
In 1816, Orazioni di Stefano Porcari (Speeches by 15th-century political opponent of civic Papal authority); in 1818, he wrote Discorso sopra gli spettacoli, le feste ed i lusso degli Italiani nel secolo XIV.
In 1815, in honor of his patron, he edited Regimento dei costumi delle donne written by Francesco da Barberino.
He published a treatise on painting, putatively written by Leonardo da Vinci, and collected by Francesco Melzi.
He also edited in 1817 Trattati della compunzione del cuore di San Giovanni Grisostomo.