John Pym (20 May 1584 – 8 December 1643) was a politician from Somerset who played a key role in establishing the modern English Parliamentary system. One of the Five Members whose attempted arrest in January 1642 was a major step in sparking the First English Civil War, his use of procedure to outmanoeuvre opponents was unusual for the period. Though this meant he was respected by contemporaries rather than admired, in 1895 historian Goldwin Smith described him as "the greatest member of Parliament that ever lived".

Pym's father died when he was seven months old, and he was raised by his stepfather Sir Anthony Rous, from whom he inherited his Puritan views and deep opposition to Archbishop Laud's reforms. He was also a leading member of the Providence Island Company, which attempted to establish a Puritan colony in Central America.

Described as 'a true revolutionary', he led the opposition to arbitrary rule first under James I, then Charles I. His leadership in the early stages of the war was essential to the Parliamentarian cause, particularly his role in negotiating the Solemn League and Covenant with the Scots Covenanters; his death from cancer in December 1643 was considered a major blow.

Originally buried in Westminster Abbey, after the Stuart Restoration in 1660, his body was dumped in a pit at nearby St Margaret's, Westminster along with those of other Parliamentary leaders. Though his reputation later suffered in comparison to less complex figures like John Hampden and Viscount Falkland, he is now viewed as an astute politician and effective speaker. Many of his ideas were adopted by Patriots during the American Revolution and 19th-century American liberals.

His father, Alexander Pym (1547–1585), was a member of the minor gentry from Brymore in Somerset, and a successful lawyer in London, where John was born in 1584. Alexander died seven months later and his mother, Philippa Colles (died 1620), married a wealthy Cornish landowner, Sir Anthony Rous. A close friend and executor of Sir Francis Drake, Sir Anthony instilled in his stepson a strong dislike of Spain, a zealous Puritanism, and opposition to both the Catholic Church, and the rival Protestant theology of Arminianism.

The Rous were a large and close-knit family, often wedding relatives and friends. In May 1604, Pym married Anne Hooke, a daughter of Barbara Rous and John Hooke, and aunt of the scientist Robert Hooke. Before her death in 1620, they had seven children, of whom four survived into adulthood; Philippa (1604–1654), Charles (1615–1671), Dorothy (1617–1661), and Catherine.

Pym was educated at Pembroke College, Oxford, then known as Broadgates and famous for 'advanced Protestantism'. Since legal knowledge was considered part of an education, he subsequently attended the Middle Temple in 1602; while he does not appear to have formally graduated from either, he made a number of lifelong friends, the most important of which being William Whitaker.

In June 1605, he was appointed collector of taxes for the Exchequer in Hampshire, Gloucestershire, and Wiltshire; this gave him a broader range of connections than many contemporaries, who were often confined to family or county networks. Whitaker's father had been Member of Parliament for Westbury, Wiltshire, and in 1621, Pym was elected for the nearby seat of Calne.