Pope Gregory X (Latin: Gregorius X; c. 1210 – 10 January 1276), born Teobaldo Visconti, was head of the Catholic Church and ruler of the Papal States from 1 September 1271 to his death and was a member of the Secular Franciscan Order.

As to Gregory's regulations on the conduct of the conclave, though briefly annulled by Adrian V and John XXI, they remained in force until the 20th century.

It is said that he began his career by attaching himself to the household of the Cistercian Cardinal Giacomo di Pecorari, Bishop of Palestrina (1231–1244), who was also from Piacenza.

[5] He then returned to the Cardinal, whose new assignment in France was actually to preach a crusade against Frederick II, the Hohenstaufen Emperor, who was again at war with Pope Gregory IX.

In 1243, when the Bishop of Piacenza died, Innocent IV offered the position to Archdeacon Teobaldo, who declined, preferring to follow in the company of Cardinal Giacomo.

Upon the death of his patron and spiritual model, Teobaldo decided not to remain any longer in the Curia, planning to travel to Paris, where he would study theology.

[9] During this period, Visconti became acquainted with people such as Bonaventure, Thomas Aquinas, Gui Foucois, Pierre de Tarentaise and Matteo Rubeo Orsini, all of whom were participants in the council.

[10] He was instructed by Pope Innocent IV (Sinibaldo Fieschi, 1243–1254) to preach the crusade for the recovery of the Holy Land.

The Provost of Utrecht, the candidate of Frederick II, had attempted to usurp the seat, and Cardinal Giacomo de Pecorari, Bishop of Palestrina had been ordered by the Pope to intervene and prohibit an election until the canons of the cathedral could meet with him.

The new Bishop-Elect and his Archdeacon immediately had problems about Teobaldo's conduct of his office while being absent in Paris, an issue which was not settled until the end of 1250.

During a Chapter meeting, he was roundly criticized by Teobaldo, who accused him of compromising the virtue of Berta, daughter of Conrardi de Coen le Frison, by force.

Visconti was elected to succeed Pope Clement IV on 1 September 1271 after the papal chair had been vacant for two years and nine months, due to divisions among the cardinals.

Charles had also successfully intrigued to have himself elected Senator of Rome, and he repeatedly intervened in the political affairs of the entire Italian peninsula.

On 6 January 1266, he was crowned in St. Peter's Basilica by Cardinals Riccardo Annibaldi, Raoul de Grosparmy, Ancher Pantaleoni, Matteo Orsini, and Goffredo da Alatri, with permission of Pope Clement IV, who did not dare to approach Rome himself due to the hostility of the Ghibelline government toward him.

[18] The deadlock was not even broken when the citizens of Viterbo locked up the cardinals in the Episcopal palace where they were meeting, and finally tearing off part of the roof of the building.

Finally, in August 1271, the Cardinals decided to appoint a committee, three of their number from each side, to negotiate a settlement (Election by Compromise).

[19] This was a victory, in fact, for the French-leaning faction, since Teobaldo had intimate connections with France, and his nephew, Vicedomino de Vicedomini, a native of Piacenza but Archbishop of Aix in Provence, had been a follower and advisor of Charles of Anjou ever since he came into Italy.

The election of Visconti, after a 2-year, 9-month struggle, came as a complete surprise to him, since it took place while he was engaged in the Ninth Crusade at Acre in Palestine with King Edward I of England.

On 1 January 1272, the Pope-Elect reached Brindisi, and he arrived in Viterbo, the site of the Election, where the Cardinals were waiting, in early February 1272.

As soon as he was elected in 1271, Pope Gregory X received a letter from the Mongol Great Khan Kublai, remitted by Niccolò and Maffeo Polo following their travels to his court in Mongolia.

[21] After the council, Abaqa sent another embassy led by the Georgian Vassali brothers to further notify Western leaders of military preparations.

Two days after his coronation Gregory X sent a letter to King Edward I of England, inviting him to a general council to be held on the subject of the Holy Land, beginning on 1 May 1274.

[29] The Second Ecumenical Council met at Lyons, beginning on 1 May 1274 for the purpose of considering the East-West Schism, the condition of the Holy Land, and the abuses of the Catholic Church.

[32] In connection with the financing of the Crusade, Pope Gregory imposed on all clerics a six-year long tax of 10% of their income.

[33] His Constitution with respect to conclaves legislated a number of important points:[34] Pope Gregory's health had worsened after he left Lyons in April 1275.

Writing in L'Osservatore Romano, Agostino Paravicini Bagliani says that the popular belief that Pius V (1566–72) was the first Pope to wear the white cassock is inaccurate.

Teobaldo Visconti is a central character in the 1982 American-Italian miniseries Marco Polo, in which he is portrayed by American actor Burt Lancaster.