James Harrington (or Harington; 3 January 1611 – 11 September 1677) was an English political theorist of classical republicanism. He is best known for his controversial publication The Commonwealth of Oceana (1656). This work was an exposition of an ideal constitution, a utopia, designed to facilitate the development of the English republic established after the regicide, the execution of Charles I of England in 1649.

Harrington was born in 1611 in Upton, Northamptonshire. He was the eldest son of Sir Sapcote(s) Harrington of Rand, Lincolnshire, who died in 1630, and his first wife, Jane Samwell of Upton, daughter of Sir William Samwell. James Harrington was the great-nephew of John Harington, 1st Baron Harington of Exton, who died in 1613. For a period, he resided with his father in the manor house at Milton Malsor, Northamptonshire, which had been bequeathed by Sir William Samwell to his daughter following her marriage. A blue plaque on Milton Malsor Manor commemorates this residence.

Holy Cross Church in Milton Malsor contains a monument on the south wall of the chancel to Harrington's mother, Dame Jane Harrington. According to the memorial, she died on 30 March 1619, when James was 7 or 8 years old. The memorial reads, in modern English but punctuated as in the original:

"Here under lies the body of Dame Jane, daughter of Sir William Samwell Knight, & late wife to Sir Sapcotes Harington [sic] of Milton Knight, by whom he had issue 2 sons & 3 daughters, viz James, William, Jane, Anne & Elizabeth. Which Lady died March 30, 1619".

When his father died in June 1630, James commissioned a second monument, which can still be seen in the Church of St Oswald at Rand, Lincolnshire. It depicts Sapcotes and his first wife Jane together with their five children.

Knowledge of Harrington's childhood and early education is thin, though he clearly spent time both in Milton Malsor and at the family manor in Rand. In 1629 he entered Trinity College, Oxford as a gentleman commoner and left two years later with no degree. His eighteenth-century biographer, John Toland, says that while there one of his tutors was the royalist High Churchman William Chillingworth, which may have been the case before the latter left for the Catholic seminary in Douai in 1630. On 27 October 1631 Harrington entered the Middle Temple.

By this time, Harrington's father had died, and his inheritance helped pay his way through several years of continental travel. There is some suggestion that he enlisted in a Dutch militia regiment (apparently seeing no service), before touring the Netherlands, Denmark, Germany, France, Switzerland and Italy. He was in Geneva with James Zouche in the summer of 1635 and subsequently travelled to Rome. In the light of this, Toland's reference to his visiting the Vatican, where he "refused to kiss the Pope's foot", probably refers to early 1636; meanwhile his visit to Venice helped bolster his knowledge of, and enthusiasm for, the Italian republics. The following decade, including his comings and goings during the Civil Wars, are largely unaccounted for by anything other than unsubstantiated stories, for example that he accompanied Charles I to Scotland in 1639 in connection with the first Bishops' War. In 1641–42 and in 1645 he provided financial assistance to Parliament, providing loans and perhaps also collecting money on behalf of Parliament in Lincolnshire. Yet, around the same time, he was acting as 'agent' for Charles Louis, the Prince Elector Palatine, who was nephew of Charles I and whose brother Prince Rupert led the Royalist forces in the English Civil War. Charles Louis and his mother had declared their support for Parliament in 1642.

Harrington's apparent political loyalty to Parliament did not interfere with a strong personal devotion to the King. Following the capture of Charles I, Harrington accompanied a "commission" of MPs appointed to accompany Charles in the move from Newcastle to Holdenby House (Holmby), after he had been relinquished by the Scots, who had captured him. Harrington's cousin Sir James Harrington was one of the Commissioners, which perhaps explains why the future author of Oceana was one of those who accompanied the commissioners as servants 'to wait upon' the King on the journey. Harrington continued as 'gentleman of the bedchamber' to the King once they reached Holdenby House, and we see him acting in that capacity through to the end of the year at both Carisbrooke Castle and Hurst Castle. However, while at Hurst Castle Harrington got into a discussion with the Governor and various army officers during which he voiced his support for the King's position concerning the Treaty of Newport, resulting in his dismissal.