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
- Saint Leo of Patara (d. 260), early Christian martyr
- Emperor Leo the Great (d. 474), Roman emperor of the East, venerated in Eastern Orthodoxy
This set index article includes a list of related items that share the same name (or similar names).