Alexander III, né Orlando Bandinelli (d. August 3, 1181), was pope from 1159 to 1181.
He was born in Siena, and first made his mark as teacher of canon law at the University of Bologna, where he composed the Stroma or the Summa Magistri Rolandi, one of the earliest commentaries on the Decretum Gratiani.
In October 1150 Pope Eugene III created him cardinal deacon of SS. Cosmae and Damiani; later he became cardinal priest of St Mark's. Probably about this time he composed his Sentences, based on the Introductio ad theologiam of Pierre Abélard. In 1153 he became papal chancellor, and was the leader of the cardinals opposed to Frederick Barbarossa.
On the September 7, 1159 he was chosen the successor of Pope Adrian IV, a minority of the cardinals, however, electing the cardinal priest Octavian, who assumed the name of Victor IV. This antipope, and his successors Paschal III (1164-1168) and Calixtus III (1168-1178), had the imperial support; but after the defeat of Legnano, Barbarossa finally (in the peace of Venice, 1177) recognized Alexander as pope. On the 12th of March 1178 Alexander returned to Rome, which he had been compelled to leave twice, namely, from 1162 until the 23rd of November 1165, and again in 1167. The first period he spent in France, the latter chiefly in Gaeta, Benevento, Anagni and Venice.
In March 1179 Alexander held the Third Council of the Lateran, a brilliant assemblage, reckoned by the Roman church as the eleventh ecumenical council; its acts embody several of the pope's proposals for the betterment of the condition of the church, among them the present law requiring that no one may be elected pope without the votes of two-thirds of the cardinals. This synod marks the summit of Alexander's power. Besides checkmating Barbarossa, he had humbled Henry II of England in the affair of Thomas Becket, he had confirmed the right of Alphonso I of Portugal to the crown, and even as a fugitive had enjoyed the favour and protection of Louis VII of France. Nevertheless, soon after the close of the synod the Roman republic forced Alexander to leave the city, which he never re-entered; and on September 29, 1179, some nobles set up the antipope Innocent III. By the judicious use of money, however, Alexander got him into his power, so that he was deposed in January 1180. In 1181 Alexander excommunicated William the Lion of Scotland and put the kingdom under the interdict. The great pope died at Civita Castellana on the 3rd of August 1181.