Nathaniel Fiennes (c. 1608 – 16 December 1669) was an English politician who sat in the House of Commons at various times between 1640 and 1659. He was an officer in Parliamentary army during the English Civil War and an active supporter of the republican cause during the Interregnum.
He was the second son of William Fiennes, 1st Viscount Saye and Sele, by Elizabeth, daughter of John Temple, of Stowe in Buckinghamshire, was born in 1607 or 1608, and educated at Winchester and at New College, Oxford, where, as founder's kin, he was admitted a perpetual fellow in 1624.
After about five years residence he left without taking a degree, travelled abroad, and in Switzerland imbibed or strengthened those religious principles and that hostility to the Laudian church which were to be the chief motive in his future political career. He returned to Scotland in 1639, and established communications with the Covenanters and the Opposition in England. As Member of Parliament for Banbury in both the Short and Long Parliaments he took a prominent part in the attacks upon the church.
He spoke against the illegal canons on 14 December 1640, and again on 9 February 1641 on the occasion of the reception of the London petition, when he argued against episcopacy as constituting a political as well as a religious danger and made a great impression on the House of Commons, his name being added immediately to the committee appointed to deal with church affairs.
He took a leading part in the examination into the army plot; was one of the commissioners appointed to attend the king to Scotland in August 1641; and was nominated one of the committee of safety in July 1642. On the outbreak of hostilities he took arms immediately, commanded a troop of horse in the army of Lord Essex, was present at the relief of Coventry in August, and at the fight at Powick Bridge, Worcester in September, where he distinguished himself, and subsequently at Edgehill. Of the last two engagements he wrote accounts, viz. True and Exact Relation of both the Battles fought by ... Earl of Essex against the Bloudy Cavaliers (1642). (See also A Narrative of the Late Battle before Worcester taken by a Gentleman of the Inns of Court from the mouth of Master Fiennes, 1642).