John Thornhagh (1648–1723) was an English Member of Parliament.
He was the only son of Francis Thornhagh, MP for East Retford and educated at Jesus College, Cambridge (1664). He succeeded his father in 1648.
He was appointed High Sheriff of Nottinghamshire for 1688–89. He was elected MP for East Retford in 1689, sitting until 1702. He later served as knight of the shire (MP) for Nottinghamshire from 1704 to 1710. He was a Justice of the Peace and a Deputy Lieutenant for the county.
He married Elizabeth, the daughter of Sir Richard Earle, 1st Baronet, of Stragglethorpe, Lincolnshire, with whom he had 3 sons and 5 daughters. He was succeeded by his son St Andrew Thornhagh.