St. Leo may refer to one of several saints named Leo, including:
Pope Leo I (d. 461), pope and saint
Pope Leo II (d. 683), pope and saint
Pope Leo III (d. 816), pope and saint
Pope Leo IV (d. 855), pope and saint
Pope Leo IX (d. 1054), pope and saint
Saint Leo of Bayonne, France
Saint Leo of Catania otherwise Saint Leo the Thaumaturge (d. 785), saint and bishop of Catania in Sicily
Saint Leo of Montefeltro (d. 366), bishop of Montefeltro