Ranulf Flambard, or Ralph (died September 5, 1128) was Bishop of Durham and a government minister of William Rufus.

He was the son of a Norman parish priest in the diocese of Bayeux. Emigrating at an early age to England, the young Ranulf entered the chancery of William I and became conspicuous as a courtier. He was disliked by the barons, who nicknamed him Flambard in reference to his talents as a mischief-maker; but he acquired the reputation of an acute financier and appears to have played an important part in the compilation of the Domesday survey. In that record he is mentioned as a clerk by profession, and as holding land both in Hants and Oxfordshire. Before the death of the old king he became chaplain to Maurice, bishop of London, under whom he had formerly served in the chancery.

Early in the next reign Ranulf returned to the royal service. He is usually described as the chaplain of Rufus, but he is also called treasurer and his services were chiefly of a fiscal character. His name is regularly connected by the chroniclers with the rapacious extortion from which all classes suffered between 1087 and 1100. He profited largely by the tyranny of Rufus, farming for the king a large proportion of the ecclesiastical preferments which were illegally kept vacant. He personally managed sixteen abbeys or bishoprics and obtaining for himself the wealthy see of Durham (1099).

His fortunes fell upon the accession of Henry I, by whom he was imprisoned in the Tower of London. Flambard soon escaped, and is noted as not only the first inmate of this soon-to-be-infamous prison, but also the first person to escape from it. A popular legend represents the bishop as descending from the window of his cell by a rope which friends had conveyed to him in a cask of wine. He took refuge across the English Channel with Henry's brother Robert, Duke of Normandy.

As Robert's advisor, he pressed the duke to dispute his brother Henry's claim to the crown of England. Robert invaded England in 1101, but he agreed at the Treaty of Alton to renounce his claim to the English throne.

Robert rewarded the bishop by entrusting him with the administration of the see of Lisieux. After Robert's defeat by Henry at Tinchebray in 1106, the bishop was among the first to make his peace with Henry, and was returned to the see he had purchased in 1099. He retired from political life and Henry found in Roger of Salisbury an able financier who was infinitely more acceptable to the nation.

At Durham he passed the remainder of his life. His private life scandalized the local clergy; he had at least two sons, for whom he purchased benefices before they had entered on their teens; and scandalous tales are told of the entertainments with which he enlivened his seclusion. But he distinguished himself as a builder and a pious founder. He all but completed the cathedral which his predecessor, William of St. Carilef, had begun; fortified Durham; built Norham Castle ; founded the priory of Mottisfout and endowed the college of Christchurch, Hampshire.

|- style="text-align: center;" | width="30%" |Preceded by:
Waldric | width="40%" style="text-align: center;" |Lord Chancellor
1107–1123 | width="30%" |Succeeded by:
Geoffrey Rufus