Lanfranc OSB (1005 x 1010 – 24 May 1089) was an Italian-born English churchman, monk and scholar. Born in Italy, he moved to Normandy to become a Benedictine monk at Bec. He served successively as prior of Bec Abbey and abbot of St Stephen's Abbey in Caen, Normandy and then as Archbishop of Canterbury in England, following its conquest by William the Conqueror. He is also variously known as Lanfranc of Pavia (Italian: Lanfranco di Pavia), Lanfranc of Bec (French: Lanfranc du Bec), and Lanfranc of Canterbury (Latin: Lanfrancus Cantuariensis). In his lifetime, he was regarded as the greatest theologian of his generation.

Lanfranc was born in the early years of the 11th century at Pavia, where later tradition held that his father, Hanbald, held a rank broadly equivalent to magistrate. He was orphaned at an early age.

Lanfranc was trained in the liberal arts, at that time a field in which northern Italy was famous. There is little or no evidence to support the myth that his education included much in the way of Civil Law, and none that links him with Irnerius of Bologna as a pioneer in the renaissance of its study. For unknown reasons at an uncertain date, he crossed the Alps, soon taking up the role of teacher in France and eventually in Normandy. About 1039 he became the master of the cathedral school at Avranches, where he taught for three years with conspicuous success. But in 1042 he embraced the monastic profession in the newly founded Bec Abbey. Until 1045 he lived at Bec in absolute seclusion.

Lanfranc was then persuaded by Abbot Herluin to open a school at Bec to relieve the monastery's poverty. From the first he was celebrated (totius Latinitatis magister). His pupils were drawn not only from France and Normandy, but also from Gascony, Flanders, Germany and Italy. Many of them afterwards attained high positions in the Church; one possible student, Anselm of Badagio, became pope under the title of Alexander II; another, Anselm of Bec, succeeded Lanfranc as the Archbishop of Canterbury. The favourite subjects of his lectures were the trivium of grammar, logic and rhetoric and the application of these principles to theological elucidation. In one of Lanfranc's most important works, The Commentary on the Epistles of St. Paul, he is said to have "expounded Paul the Apostle; and, wherever opportunity offered, he stated the premises, whether principal or secondary, and the conclusions of Paul's arguments in accordance with the rules of Logic".

As a result of his growing reputation Lanfranc was invited to defend the doctrine of transubstantiation against the attacks of Berengar of Tours. He took up the task with the greatest zeal, although Berengar had been his personal friend; he was the protagonist of orthodoxy at the Church Councils of Vercelli (1050), Tours (1054) and Rome (1059). To Lanfranc's influence is attributed the desertion of Berengar's cause by Hildebrand and the more broad-minded of the cardinals. Our knowledge of Lanfranc's polemics is chiefly derived from the tract De corpore et sanguine Domini, probably written c. 1060–63. Though betraying no signs of metaphysical ability, his work was regarded as conclusive and became for a while a text-book in the schools. It is often said to be the place where the Aristotelian distinction between substance and accident was first applied to explain Eucharistic change. It is the most important of the surviving works attributed to Lanfranc.

In the midst of Lanfranc's scholastic and controversial activities Lanfranc became a political force. Later tradition told that while he was Prior of Bec he opposed the non-canonical marriage of Duke William with Matilda of Flanders (1053) and carried matters so far that he incurred a sentence of exile. Apparently, their relationship was within the prohibited degrees of kindred. But the quarrel was settled when he was on the point of departure, and he undertook the difficult task of obtaining the pope's approval of the marriage. In this he was successful at the same council which witnessed his third victory over Berengar (1059), and he thus acquired a lasting claim on William's gratitude. In 1066 Lanfranc became the first Abbot of the Abbey of Saint-Étienne at Caen in Normandy, a monastery dedicated to Saint Stephen which the duke had supposedly been enjoined to found as a penance for his disobedience to the Holy See.

Henceforward Lanfranc exercised a perceptible influence on his master's policy. William adopted the Cluniac programme of ecclesiastical reform, and obtained the support of Rome for his English expedition by assuming the attitude of a crusader against schism and corruption. It was Alexander II, possibly a pupil of Lanfranc's and certainly a close friend, who gave the Norman Conquest the papal benediction—a notable advantage to William at the moment, but subsequently the cause of serious embarrassments.

When the see of Rouen next fell vacant (1067), the thoughts of the electors turned to Lanfranc. But he declined the honour, and he was appointed to the English primatial see as Archbishop of Canterbury as soon as Stigand had been canonically deposed on 15 August 1070. He was speedily consecrated on 29 August 1070. The new archbishop at once began a policy of reorganisation and reform. His first difficulties were with Thomas of Bayeux, Archbishop-elect of York, (another former pupil) who asserted that his see was independent of Canterbury and claimed jurisdiction over the greater part of the English Midlands. This was the beginning of a long running dispute between the sees of Canterbury and York, usually known as the Canterbury–York dispute.