Sir Alexander St John (died June 1657) was an English politician who sat in the House of Commons variously between 1621 and 1629. St John was a son of Oliver St John, 3rd Baron St John of Bletso and his wife Dorothy Reid, daughter of Sir John Rede or Reid, of , Gloucestershire. He was admitted fellow commoner at Queens' College, Cambridge on 9 November 1601. He was knighted on 5 August 1608 at Bletsoe together with his brother Anthony. Apart from Anthony, four other brothers, Oliver, Rowland, Beauchamp and Henry were to become MPs.