Pope Saint Agatho (c. 577 – January 10, 681) was pope from June 27, 678 to January 10, 681.

A Greek born in Sicily of wealthy and devout parents, he allegedly gave away his inheritance after their death and retired to a monastery in Palermo. This belief is based on a letter written by St. Gregory the Great to the abbot of St. Hermes in Palermo, a Benedictine Monastery, mentioning an Agatho. In this letter, Gregory wrote that the abbot could receive Agatho into his monastery if Agatho's wife was willing to enter a convent. While there are reasons to believe that Pope Agatho is this monk, he would have been over 100 years old at the time of his election.

Shortly after Agatho became Pope, St Wilfrid, Archbishop of York, arrived at Rome to invoke the authority of the Holy See in his behalf. Wilfrid had been deposed from his see by Theodore, Archbishop of Canterbury, who had carved up Wilfrid's diocese, appointing three bishops to govern the new sees. At a synod which Pope Agatho convoked in the Lateran to investigate the affair, it was decided that Wilfrid's diocese should indeed be divided, but that Wilfrid himself should name the bishops.

The major event of his pontificate was the Sixth Ecumenical Council (680–1), which ended the Monothelite heresy that had been tolerated by previous popes (Honorius among them). The council began when Emperor Constantine IV, desired to heal the schism that separated the two sides. He wrote to Pope Donus suggesting a conference on the matter, but Donus was dead by the time the letter arrived. However, Agatho was quick to seize the olive branch offered by the emperor. He ordered councils held throughout the West so that legates could present the universal tradition of the Western Church. Then he sent a large delegation to meet the Easterners at Constantinople.

The legates and patriarchs gathered in the imperial palace on November 7, 680. The Monothelites presented their case. Then the letter of Pope Agatho was read which explained the traditional belief of the Church that Christ was of two wills, divine and human. The council agreed that Peter spoke through Agatho. Patriarch George of Constantinople accepted Agatho's letter, as did most of the bishops present. The council proclaimed the existence of the two wills in Christ and condemned Monothelitism, with Pope Honorius being included in the condemnation. When the council ended in September of 681 the decrees were sent to the Pope, but Agatho had died in January. The Council had not only ended the Monothelite heresy, but had healed the schism.

Agatho also undertook negotiations between the Holy See and Constantine, concerning the relations of the Byzantine Court to papal elections. Constantine promised Agatho to abolish or reduce the tax that the popes had had to pay to the imperial treasury on their consecration.

He is venerated as a saint by both Latins and Greeks.

Some Traditionalist Catholics say he was the first pope to take, as part of his inauguration, what they call the Papal Oath.