John Gernoun, or Gernon (died c.1357) was an Irish judge who held office as Serjeant-at-law (Ireland) and Chief Justice of the Irish Common Pleas. He claimed descent from the de Gernon family who are listed as "barons" in the Domesday Book. His father Roger Gernoun was a military commander who distinguished himself during the Bruce campaign in Ireland. John was probably born in Louth, since the Gernoun family had strong links to that county and gave their name to Gernonstown: several members of the family held the office of High Sheriff of Louth between the fourteenth and seventeenth centuries.