Leo III (died June 12, 816) was Pope from 795 to 816.

Leo announced his election to Charlemagne, sending him the keys of Saint Peter's tomb and the banner of Rome, requesting an envoy. Charlemagne in his reply stated that it was his function to defend the church and the Pope's to pray for the realm and for victory of the army.

Leo aroused the hostility of the nobility, and in April 799 he was attacked by a gang. He was then formally deposed and sent to a monastery, but escaped and made his way to Charlemagne, who did not recognise the deposition. The opposition in Rome sent representatives to Charlemagne to present their case. Alcuin, Charlemagne's adviser, pointed out that no earthly power could judge the Pope, and so Leo was escorted back to Rome.

Charlemagne went to Rome in November 800, and on December 1 held a council there with representatives of both sides. Leo, on December 23, took an oath of purgation concerning the charges brought against him, and his opponents were exiled. Two days later Leo crowned Charlemagne after the latter prayed at St. Peter's tomb. Charlemagne was to intervene in church affairs, not always successfully.

Leo helped restore King Eardulf of Northumbria, and settled various matters of dispute between the Archbishops of York and Canterbury.

The reasons for the coronation, the involvement beforehand of the Frankish court, and the relationship to the Byzantine Empire are all matters of debate among historians. An effective administrator of the papal territories, Leo contributed to the beautification of Rome.

His feast day, formerly, was June 12.