John Knox (Scottish Gaelic: Iain Cnocc; born c. 1514 – 24 November 1572) was a Scottish minister, Reformed theologian, and writer who was a leader of the country's Reformation. He was the founder of the Presbyterian Church of Scotland.

Born in Giffordgate, a street in Haddington, East Lothian, Knox is believed to have been educated at the University of St Andrews and worked as a notary-priest. Influenced by early church reformers such as George Wishart, he joined the movement to reform the Scottish church. He was caught up in the ecclesiastical and political events that involved the murder of Cardinal David Beaton in 1546 and the intervention of the regent Mary of Guise. He was taken prisoner by French forces the following year and exiled to England on his release in 1549.

While in exile, Knox was licensed to work in the Church of England, where he rose in the ranks to serve King Edward VI of England as a royal chaplain. He exerted a reforming influence on the text of the Book of Common Prayer. In England, he met and married his first wife, Margery Bowes. When Mary I ascended the throne of England and re-established Catholicism, Knox was forced to resign his position and leave the country. Knox moved to Geneva and then to Frankfurt. In Geneva, he met John Calvin, from whom he gained experience and knowledge of Reformed theology and Presbyterian polity. He created a new order of service, which was eventually adopted by the reformed church in Scotland. He left Geneva to head the English refugee church in Frankfurt but he was forced to leave over differences concerning the liturgy, thus ending his association with the Church of England. The University of Edinburgh Heritage Collection holds a copy of Knox's Liturgy, translated into Scots Gaelic by John Carswell. It is the first book printed in any Gaelic language.

On his return to Scotland, Knox led the Protestant Reformation in Scotland, in partnership with the Scottish Protestant nobility. The movement may be seen as a revolution since it led to the ousting of Mary of Guise, who governed the country in the name of her young daughter Mary, Queen of Scots. Knox helped write the new confession of faith and the ecclesiastical order for the newly created reformed church, the Kirk. He wrote his five-volume The History of the Reformation in Scotland between 1559 and 1566. He continued to serve as the religious leader of the Protestants throughout Mary's reign. In several interviews with the Queen, Knox admonished her for supporting Catholic practices. After she was imprisoned for her alleged role in the murder of her husband Lord Darnley, and King James VI was enthroned in her stead, Knox openly called for her execution. He continued to preach until his final days.