Florentine conspirator who attempted in 1570–71 to overthrow Queen Elizabeth I of England in favour of Mary Stuart, Queen of Scots, who then was to be married to Thomas Howard, 4th Duke of Norfolk (Norfolk, Thomas Howard, 4th Duke of, Earl Of Surrey, Earl Marshal). Ridolfi intended to secure these results by the murder of Elizabeth and a Spanish invasion of England.

A member of the prominent Florentine family of Ridolfi di Piazza, Ridolfi was trained as a merchant and banker. He went to London as a business agent about 1555, during the reign of the Catholic queen of England, Mary I (d. 1558), whose husband, the future king Philip II of Spain, later figured in Ridolfi's plan. Although Ridolfi was trusted and employed by Elizabeth's government, his ardent Catholicism led him into political activity on behalf of discontented English Catholics. After the failure of revolts in the north of England (1569–70) in which he was involved, Ridolfi and John Leslie (Leslie, John), Catholic bishop of Ross, concluded that foreign military backing was essential. Ridolfi left England in March 1571 to obtain assistance from Pope Pius V, Philip II of Spain, and the Duke de Alba, Spanish governor-general of the Netherlands. With some difficulty he procured from the Duke of Norfolk a written statement that the duke was a Catholic and would lead an English revolution supported by Spain.

Ridolfi's plot was exposed in April 1571 when his messenger, Charles Baillie, was arrested at Dover, Kent. Baillie's confession and the letters that he was carrying incriminated many conspirators, including Leslie, who was imprisoned for two years, and Norfolk, who was executed for treason (June 2, 1572). Only Elizabeth's forbearance saved Mary Stuart, then in captivity in England, from death at that time. Ridolfi, who was still abroad when the plot was discovered, returned to Italy, becoming a Florentine senator in 1600.