Ratramnus (died circa 868) was a theological controversialist of the second half of the 9th century.

He was a monk of the Benedictine abbey of Corbie near Amiens; beyond this fact very little is known about him. He is best known for his treatise on the Eucharist (De corpore et sanguine Domini liber), in which he contradicted the doctrine of transubstantiation, taught in a similar work by his contemporary Radbertus Paschasius. Ratramnus sought to reconcile science and religion, whereas Radbertus emphasized the miraculous. Ratramnus's views failed to find acceptance; their author was soon forgotten, and, when the book was condemned as heresy at the synod of Vercelli in 1050, it was described as having been written by Johannes Scotus Erigena at the command of Charlemagne. During the Reformation, there was a revival of interest in the book; it was published in 1532 and immediately translated.

In the controversy about election, when appealed to by Charles the Bald, Ratramnus had written two books: De praedestinatione Dei, in which he maintained the doctrine of a twofold predestination; nor did the fate of Gottschalk deter him from supporting his view against Hincmar as to the orthodoxy of the expression trina Deitas. Ratramnus was famous in his own day for his Contra Graecorum opposita, in four books (868), a valued contribution to the controversy between the Eastern and Western Churches which had been raised by the publication of the encyclical letter of Photius in 867. An edition of De corpore et sanguine Domini was published at Oxford in 1859.

This article incorporates text from the public domain 1911 Encyclopædia Britannica.