Robert Catesby (3 March 1572 – 8 November 1605) was the leader of a group of English Catholics who planned the failed Gunpowder Plot of 1605. Born in Warwickshire, Catesby was educated at Oxford University. His family were prominent recusant Catholics, and presumably to avoid swearing the Oath of Supremacy he left college before taking his degree. He married a Protestant in 1593 and fathered two children, one of whom survived birth and was baptised in a Protestant church. In 1601 he took part in the Essex Rebellion but was captured and fined, after which he sold his estate at Chastleton.

The Protestant James I, who became King of England in 1603, was less tolerant of Catholics than many persecuted Recusants had hoped. Catesby therefore planned a decapitation strike which he considered tyrannicide, aimed at the Government of England; by blowing up the King and the House of Lords with gunpowder during the State Opening of Parliament. The assassination of the King was to be the prelude to a popular uprising aimed at regime change, through which a Catholic monarch would be seated upon the English throne. Early in 1604, Catesby began to recruit other Catholics to his cause, including Thomas Wintour, John Wright, Thomas Percy, and Guy Fawkes. Over the following months, Fawkes helped to recruit a further eight conspirators into the plot, which, against the pleas of underground Jesuit superior Fr. Henry Garnet to cancel the plot, was scheduled to be carried out on 5 November 1605. Concerns about possible collateral damage caused an anonymous letter of warning to be sent to William Parker, 4th Baron Monteagle, who alerted the authorities. On the night before the planned explosion, Fawkes was arrested underneath the House of Parliament while guarding 38 barrels of gunpowder. News of his arrest caused the other plotters to flee London, warning Catesby along their way.

With a much-diminished group of followers, Catesby made a last stand at Holbeche House in Staffordshire (the modern-day Kingswinford suburb of Wall Heath), against a 200-strong Sheriff's posse led by Richard Walsh. Catesby was mortally wounded by gunfire and later found dead inside Holbeche Hall, where he had died while contemplating a holy card of the Virgin Mary. As a warning to other potential regicides, Catesby's body was exhumed, posthumously executed, and his severed head on a spike was displayed outside the Houses of Parliament.

He was born after 1572, the third and only surviving son and heir of Sir William Catesby of Lapworth in Warwickshire, by his wife Anne Throckmorton, a daughter of Sir Robert Throckmorton (c.1513–1581), KG, of Coughton Court in Warwickshire (by his second wife, Elizabeth Hussey). He was a lineal descendant of William Catesby (1450–1485), the influential councillor of King Richard III who was captured at the Battle of Bosworth and executed. His parents were prominent recusant Catholics; his father had suffered years of imprisonment for his faith, and in 1581 had been tried in Star Chamber alongside William Vaux, 3rd Baron Vaux of Harrowden, and his brother-in-law Sir Thomas Tresham, for harbouring the Jesuit priest Edmund Campion. The head of the Throckmortons, Sir Thomas Throckmorton, was also fined for his recusancy, and spent many years in prison. Another relation, Sir Francis Throckmorton, had been executed in 1584 for his involvement in a plot to free Mary, Queen of Scots.

In 1586 Robert was educated at Gloucester Hall in Oxford, a college noted for its Catholic intake. Those either studying at university or wishing to take public office could not do so without first swearing the Oath of Supremacy, an act which would have compromised Catesby's Catholic faith. Presumably to avoid this consequence, he left without taking his degree, and may then have attended the seminary college of Douai. In 1588, at the time of the Spanish Armada, Robert was allegedly imprisoned at Wisbech Castle along with Francis Tresham.

On 8 May 1592, he married Katherine Leigh, daughter of Sir Thomas Leigh of Stoneleigh Abbey in Warwickshire (1530–1626) and Katherine Spencer (1544–1626), and granddaughter of Sir Thomas Leigh of Stoneleigh Abbey and his wife Alice Barker, sometimes known as Coverdale, who was an heiress, thanks to her uncle Sir Rowland Hill, publisher of the Geneva Bible.^{[better source needed]}

Katherine came from wealthy and respected Protestant families and brought with her a dowry of £2,000, but also a religious association which offered Robert some respite from the recusancy laws then in effect. On the death of his grandmother Katherine in 1593, he came into the family property at Chastleton, in Oxfordshire. The couple's first son William died in infancy, but their second son Robert survived, and was baptised at Chastleton's Anglican parish church on 11 November 1595. When Catesby's father died in 1598, his estates at Ashby St Ledgers were left to his wife for life, while Catesby and his family remained at Chastleton. Catesby had seemed happy to remain a Church Papist but after his wife's death later that year he further embraced Catholicism.

In 1601 Catesby was involved in Essex's Rebellion. The Earl of Essex's purpose might have lain in furthering his own interests rather than those of the Catholic Church, but Catesby hoped that if Essex succeeded, there might once more be a Catholic monarch. The rebellion was a failure, however, and the wounded Catesby was captured, imprisoned at the Wood Street Counter, and fined 4,000 marks (approximately equivalent to £698,000 in 2025) by Elizabeth I. Sir Thomas Tresham helped pay some of Catesby's fine and Catesby sold his estate at Chastleton afterwards. Several authors speculate about Catesby's movements as Elizabeth's health grew worse; he was probably among those "principal papists" imprisoned by a government fearing open rebellion, and in March 1603 he possibly sent Christopher Wright to Spain to see if Philip III would continue to support English Catholics after Elizabeth's death. Catesby funded the activities of some Jesuit priests, and while visiting them made occasional use of the alias Mr Roberts.