Pope Leo IV (died 17 July 855) was the bishop of Rome and ruler of the Papal States from 10 April 847 to his death in 855.

He is remembered for repairing Roman churches that had been damaged during the Arab raid against Rome, and for building the Leonine Wall around Vatican Hill to protect the city.

Pope Leo organized a league of Italian cities who fought and won the sea Battle of Ostia against the Saracens.

As the attack of the Saracens on Rome in 846 caused the people to fear for the safety of the city, he was consecrated on 10 April, 847 without waiting for the consent of the emperor.

[1] Leo ordered a new line of walls encompassing the suburb on the right bank of the Tiber to be built, including St. Peter's Basilica, which had been undefended until this time.

[3][1] In 849, when a Saracen fleet from Sardinia approached Portus, Leo IV summoned the maritime republics – Naples, Gaeta and Amalfi – to form a league.

Before his death in 855 the Pope welcomed Aethelwulf King of Wessex and his sons, including the seven year old Alfred the Great, who at 5 had already met him in 853, as pilgrims to Rome.