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.