John Fiennes (c. 1612–1708) was the third son of Puritan noble and politician William Fiennes, 1st Viscount Saye and Sele. He served in the Parliamentarian army during the First English Civil War, before resigning his commission when elected MP for Morpeth in 1645. Excluded from Parliament by Pride's Purge in December 1648, he largely avoided politics thereafter but was appointed to Cromwell's Upper House in 1658.

John Fiennes was born c. 1612, probably at the family home of Broughton Castle in Oxfordshire, third son of the Puritan peer and politician William Fiennes, 1st Viscount Saye and Sele (1582–1662), and his wife Elizabeth Temple (died 1648). He had two elder brothers James (1602–1674) and Nathaniel (c. 1608–1669), to whom he was especially close, and three sisters, Bridget, Constance and Elizabeth.^[1]

At some point after 1670, he married Susannah Hobbs (1657–1715); their fifth and only surviving son, Lawrence (1690–1742), succeeded his cousin as Fifth Viscount Saye and Sele in January 1710, by which date Fiennes had died.^[1]

At the outbreak of the First English Civil War he joined the Parliamentarian side, and rose to the rank of the colonel of a regiment of horse by 1643, and served at the Battle of Naseby.

He was elected to the Long Parliament as member for Morpeth, probably in 1645, but was one of the MPs excluded in Pride's Purge in December 1648. In 1658, he was chosen a member of Cromwell's Upper House.