Sir John Marley or Marlay (1590–1673) was an English merchant, military commander and politician of the seventeenth century. He is best remembered for his heroic defence of Newcastle upon Tyne during the English Civil War, when he held the town for seven months against a besieging army on behalf of King Charles I. In poverty and desperation, he later betrayed the Royalist cause which he had served so loyally, and as a result, to the end of his life, he was reviled by many of his former comrades as a traitor.
His name is commemorated by Marlay House and Park near Dublin city, which belonged to a branch of his family who had settled in Ireland.