Sir John Pettus (1613–1685) was an English royalist, politician and natural philosopher.
He was the third son of Sir Augustine Pettus of Rackheath, Norfolk, by his second wife, Abigail, third daughter of Sir Arthur Heveningham of Heveningham, Suffolk. He matriculated at Pembroke College, Cambridge in 1632. He entered the service of Charles I in 1639, and was knighted on 25 November 1641, as a mark of the king's favour to Sir Richard Gurney, his father-in-law. Taken prisoner during the First English Civil War by Oliver Cromwell at Lowestoft, he was exchanged after 14 months' confinement in Windsor Castle.
Pettus then raised a regiment of horse, at his own expense; but moved on to garrison work, at Bath and Bristol. At the 1645 siege of Bristol his life was saved by Colonel Charles Fleetwood, a relation by marriage. Four charges were brought against him by the parliamentary committees of Norfolk and Suffolk, to two of which he gave satisfactory answers on his examination by the committee of sequestrations in September 1645. In November 1646 the remaining two charges were still unheard. In that year, however, he compounded, receiving support from Charles Fleetwood, whose friendship for him then caused Pettus to be suspected of disloyalty to the royalist cause. He took part in attempts to save the life of Charles I, and had to sell some of his property to meet the expenses.
After the king's execution Pettus supplied Charles II with money from time to time. He was confined by John Bradshaw for corresponding with the young king Charles, but after examination by the Council of State he was set free on bail of £4,000. In August 1651 he was assessed again but he was deep in debt, and paid only £40. In 1655 he addressed a petition to Cromwell, expressing fidelity to his government, and became deputy governor of the royal mines.