Paul the Deacon (c. 720 – 13 April probably 799), also known as Paulus Diaconus, Warnefred and Cassinensis, (i.e. "of Monte Cassino"), was a Benedictine monk and historian of the Lombards.

Life
The chief work of Paul is his Historia gentis Langobardorum. This incomplete history in six books was written after 787 and at any rate no later than 795/96, maybe at Montecassino. It covers the story of the Lombards from 568 to the death of King Liutprand in 747, and contains much information about the Byzantine empire, the Franks, and others. The story is told from the point of view of a Lombard patriot and is especially valuable for the relations between the Franks and the Lombards. It begins:
The region of the north, in proportion as it is removed from the heat of the sun and is chilled with snow and frost, is so much the more healthful to the bodies of men and fitted for the propagation of nations, just as, on the other hand, every southern region, the nearer it is to the heat of the sun, the more it abounds in diseases and is less fitted for the bringing up of the human race.
Among his sources, Paul used the document called the Origo gentis Langobardorum, the Liber pontificalis, the lost history of Secundus of Trent, and the lost annals of Benevento; he made a free use of Bede, Gregory of Tours and Isidore of Seville.
Cognate with this work is Paul's Historia Romana, a continuation of the Breviarium of Eutropius. This was compiled between 766 and 771, at Benevento. The story runs that Paul advised Adelperga to read Eutropius. She did so, but complained that this heathen writer said nothing about ecclesiastical affairs and stopped with the accession of the emperor Valens in 364; consequently Paul interwove extracts from the Scriptures, from the ecclesiastical historians and from other sources with Eutropius, and added six books, thus bringing the history down to 553. This work has little value, although it was very popular during the Middle Ages. It has been edited by H Droysen and published in the Monumenta Germaniae Historica. Auctores antiquissimi, Band ii. (1879) as well as by A. Crivellucci, in Fonti per la storia d' Italia, n. 51 (1914).
Paul wrote at the request of Angilram, bishop of Metz (d. 791), a history of the bishops of Metz to 766, the first work of its kind north of the Alps. This Gesta episcoporum Mettensium is published in Band ii. of the Monumenta Germaniae historica Scriptores, and has been translated into German (Leipzig, 1880). He also wrote many letters, verses and epitaphs, including those of Duke Arichis and of many members of the Carolingian family. Some of the letters are published with the Historia Langobardorum in the Monumenta; the poems and epitaphs edited by Ernst Dümmler will be found in the Poetae latini aevi carolini, Band i. (Berlin, 188f). Fresh material having come to light, a new edition of the poems (Die Geschichte des Paulus Diaconus) has been edited by Karl Neff (Munich, 1908), who denies, however, the attribution to Paul of the most famous poem in the collection, the Ut queant laxis, a hymn to St. John from the initial syllables of the first verses of which Guido d'Arezzo took the names of the first notes of the musical scale. Paul also wrote an epitome, which has survived, of Sextus Pompeius Festus' De significatu verborum. It was dedicated to Charlemagne.
While in France, Paulus was requested by Charlemagne to compile a collection of homilies. He executed this after his return to Monte Cassino, and it was largely used in the Frankish churches. A life of Pope Gregory the Great has also been attributed to him.