Pope Alexander III (c. 1100/1105 – 30 August 1181), born Roland (Italian: Rolando), was head of the Catholic Church and ruler of the Papal States from 7 September 1159 until his death in 1181.

A native of Siena, Alexander became pope after a contested election, but had to spend much of his pontificate outside Rome while several rivals, supported by Holy Roman Emperor Frederick I Barbarossa, claimed the papacy. Alexander rejected Byzantine Emperor Manuel I Komnenos' offer to end the East–West Schism, sanctioned the Northern Crusades, and held the Third Council of the Lateran. He canonized Thomas Becket and Bernard of Clairvaux. The city of Alessandria in Piedmont is named after him.

Rolando was born in Siena. From the 14th century, he was referred to as a member of the aristocratic family of Bandinelli, although this has not been proven. He was long thought to be the 12th-century canon lawyer and theologian Master Roland of Bologna, who composed the "Stroma" or "Summa Rolandi"—one of the earliest commentaries on the Decretum of Gratian—and the "Sententiae Rolandi", a sentence collection displaying the influence of Pierre Abélard, but John T. Noonan and Rudolf Weigand have shown this to be another Rolandus.

Rolando probably studied at Bologna, where Robert of Torigni notes that he taught theology. In October 1150, Pope Eugene III created him Cardinal-Deacon of Santi Cosma e Damiano. Later he became Cardinal-Priest of St Mark. In 1153, he became papal chancellor and was the leader of the cardinals opposed to Emperor Frederick I Barbarossa. In 1156, he negotiated the Treaty of Benevento, which restored peaceful relations between Rome and the Kingdom of Sicily. At the diet of Besançon in October 1157, he and Cardinal Bernard of San Clemente delivered a papal message that seemed to claim that Frederick I was a papal vassal, resulting in a serious diplomatic incident.

Pope Adrian IV died 1 September 1159. On 7 September 1159, Rolando Bandinelli was elected pope and took the name Alexander III. A minority of the cardinals, however, elected the cardinal priest Octavian, who assumed the name of Victor IV and became Frederick's antipope.

By 1160, faced with a papal schism, Alexander, and Victor, were summoned by Emperor Frederick to a council in Pavia. Alexander refused, stating a pope can only be judged by God. The council of Pavia met from 5 to 11 February and recognized Victor IV's election as pope. On 13 February, Victor IV excommunicated Alexander. Upon receiving the news on 24 March 1160, Alexander, who was at Anagni, excommunicated both Victor and Frederick.

In 1161, King Géza II of Hungary signed an agreement and recognised Alexander III as the rightful pope and declared that the supreme spiritual leader was the only one who could exercise the rite of investiture. This meant that Alexander's legitimacy was gaining strength, as soon proved by the fact that other monarchs, such as the king of France and King Henry II of England, recognized his authority. Because of imperial strength in Italy, Alexander was forced to reside outside Rome for a large part of his pontificate. When news reached him of the death of Victor in 1164, he openly wept, and scolded the cardinals in his company for rejoicing at the end of the rival antipope.

However, the dispute between Alexander III, Victor IV and Victor's successors Paschal III and Calixtus III (who had the German imperial support) continued until Frederick Barbarossa's defeat at the Legnano in 1176, after which Barbarossa finally (in the Peace of Venice of 1177) recognized Alexander III as pope. On 12 March 1178, Alexander III returned to Rome, which he had been compelled to leave twice: the first time between 1162 and 23 November 1165. When Alexander was arrested by supporters of the imperialist Victor IV, he was freed by Oddone Frangipane, who sent him to safety in the Campania. Alexander again left Rome in 1167, travelling first to Benevento and later moving to various strongholds such as those of Anagni, Palestrina, Ferentino, Tusculum, and Veroli.