William Petre, 4th Baron Petre (1626 – 5 January 1684) was an English peer and victim of the Popish Plot. Petre was the eldest son of Robert Petre, third Baron Petre (1599–1638), and Mary (1603–1685), daughter of Anthony-Maria Browne, second Viscount Montagu, who had been arrested in connection with the Gunpowder Plot in 1605.