Pope Eugene III (Latin: Eugenius III; c. 1080 – 8 July 1153), born Bernardo, called Bernardo da Pisa, was head of the Catholic Church and ruler of the Papal States from 15 February 1145 to his death in 1153. He was the first Cistercian to become pope. In response to the fall of Edessa to the Muslims in 1144, Eugene proclaimed the Second Crusade. The crusade failed to recapture Edessa, which was the first of many failures by the Christians in the crusades to recapture lands won in the First Crusade. He was beatified in 1872 by Pope Pius IX.

Bernardo was born in the vicinity of Pisa. Little is known about his origins and family except that he was son of a certain Godius. From the 16th century he is commonly identified as member of the family of Paganelli di Montemagno, which belonged to the Pisan aristocracy, but this has not been proven and contradicts earlier testimonies that suggest he was a man of rather humble origins. In 1106 he was a canon of the cathedral chapter in Pisa and from 1115 is attested as subdeacon. 1133–1138 he acted as vicedominus of the archdiocese of Pisa.

Between May 1134 and February 1137 he was ordained to the priesthood by Pope Innocent II, who resided at that time in Pisa. Under the influence of Bernard of Clairvaux he entered the Cistercian Order in the monastery of Clairvaux in 1138. A year later he returned to Italy as leader of the Cistercian community in Scandriglia. In Autumn 1140, Innocent II named him abbot of the monastery of S. Anastasio alle Tre Fontane outside Rome. Some chronicles indicate that he was also elevated to the College of Cardinals, but these testimonies probably resulted from a confusion because Bernardo is not attested as cardinal in any document and from the letter of Bernard of Clairvaux addressed to the cardinals shortly after his election it clearly appears that he was not a cardinal.

Bernardo was elected pope on 15 February 1145, the same day as the death of his predecessor Lucius II. Lucius had unwisely decided to take the offensive against the Roman Senate and was killed by a "heavy stone" thrown at him during an attack on the Capitol. Bernardo took the pontifical name Eugene III. He was "a simple character, gentle and retiring - not at all, men thought, the material of which Popes are made". He owed his elevation partly to the fact that no one was eager to accept an office the duties of which were at the time so difficult and dangerous and because the election was "held on safe Frangipani territory".

Bernardo's election was assisted by his being a pupil and friend of Bernard of Clairvaux, the most influential ecclesiastic of the Western Church and a strong promoter of the temporal authority of the popes. The choice did not have the approval of Bernard, however, who remonstrated against the election, writing to the entire Curia:

"May God forgive you what you have done! ... What reason or counsel, when the Supreme Pontiff was dead, made you rush upon a mere rustic, lay hands on him in his refuge, wrest from his hands the axe, pick or hoe, and raise him to a throne?"

Bernard was equally forthright in his views directly to Eugene, writing:

"Thus does the finger of God raise up the poor out of the dust and lift up the beggar from the dunghill that he may sit with princes and inherit the throne of glory."