Elizabeth Onslow (1692 – 19 April 1731) was an English aristocrat and social reformer. She was the daughter of the merchant John Knight, and niece to Colonel Charles Knight, both of whom derived great fortunes from trading in Jamaica. She was heir to both men, whose wealth derived from trading and slave plantations in Jamaica. She was married to Thomas Onslow, 2nd Baron Onslow, at St Paul's Cathedral on 17 November 1708. He may have used her great wealth to contribute to the rebuilding of his family home, Clandon Park House, as a fashionable Palladian-style mansion.