Pope Saint Leo III (750 – 12 June 816) was Pope from 795 to his death in 816. Protected by Charlemagne from his enemies in Rome, he subsequently strengthened Charlemagne's position by crowning him as Roman Emperor.

Leo, who was of non-noble birth, had risen in the hierarchy of Rome and was elected Pope only one day after the burial of his predecessor, Pope Adrian I, who had worked for good relations between Rome and the Frankish Empire under Charlemagne. Leo announced his election to Charlemagne, sending him the keys of Saint Peter's tomb and the banner of Rome, requesting an envoy. Charlemagne's reply stated that it was his function to defend the Church, and the function of the Pope to pray for the realm and for the victory of his army.

Leo aroused the hostility of Rome's nobility, who saw the papal post as reserved only for noble candidates. During his rule, he was accused of adultery and perjury. In April 799, he was attacked by a gang who unsuccessfully attempted to gouge out his eyes and cut off his tongue for his earlier actions, only to be saved by Magnus Forteman and 700 Frisian nobles of his army. He was then formally deposed and sent to a monastery, but escaped and made his way to Paderborn, where he met Charlemagne.

Charlemagne ordered Leo's enemies to Paderborn, but no decision could be found. He then had Leo escorted back to Rome. In November 800, Charlemagne himself went to Rome, and on 1 December held a council there with representatives of both sides. Leo, on 23 December, took an oath of purgation concerning the charges brought against him, and his opponents were exiled.