Roberto Ridolfi, an Italian conspirator, born in Florence about 1520. He settled in London as a merchant and banker in 1554, acted there as secret agent for the pope and other continental princes, and was engaged in several conspiracies against Queen Elizabeth. In 1569 he was imprisoned for a month and fined. In 1571 he visited Brussels, Paris, Rome, and Madrid, bearing credentials of disputed authenticity from Mary queen of Scots and the duke of Norfolk, empowering him to solicit aid to dethrone Elizabeth, and a papal decree annulling the forced marriage of Mary with Bothwell. The duke of Alva received him coldly; the pope gave him money and recommended him to Philip II. of Spain. At Madrid, where he arrived July 3, 1571, he laid before the king and six of his chosen councillors a plan for assassinating Queen Elizabeth, which he declared to have been entertained by the English Catholics and approved by the pope. This assertion, to which the pope's letter of commendation gave some color, induced the king to give a partial assent to the plot in spite of Alva's urgent objections. But meanwhile the intrigue was discovered in England and the chief conspirators were brought to punishment.

In December, 1874, on occasion of the Gladstone-Manning controversy, Lord Acton accused Pius V. of complicity in the contemplated assassination of Queen Elizabeth, while writers on the opposite side have labored to show that the pope merely approved of her being dethroned, but knew nothing of the plot against her life. Ridolfi, after the death of the duke of Norfolk and of Mary queen of Scots, continued his intrigues on the continent; but little is known of his subsequent career.