pope from 772 to 795 whose close relationship with the emperor Charlemagne symbolized the medieval ideal of union of church and state in a united Christendom.

Having been born an aristocrat and having served Popes Paul I and Stephen III (IV), he was elected pope on February 1 with the support of the Frankish party at Rome. As pope, Adrian invoked Frankish aid against the Lombard king Desiderius, who had attacked the papal possessions and was threatening Rome. By Easter 774, Charlemagne was in Rome, having destroyed the Lombard kingdom. Thenceforth, Adrian's policies were determined by the Frankish alliance rather than by relations with the Byzantine emperors of Constantinople.

The relationship between Charlemagne and Adrian was characterized by amicable rivalry. Charlemagne used the church to hold his empire together and to enforce overlordship on the Papal States, while Adrian fought firmly but adroitly for ecclesiastical autonomy and painstakingly pieced together a papal domain that was not lost until the 19th century. Despite generally warm relations and frequent cooperation in religious matters, Charlemagne's extension of his authority into Italy remained a source of tension with Adrian.

Adrian strongly opposed Adoptionism, the doctrine of the dual sonship of Christ, and condemned the teachings of Archbishop Elipandus of Toledo, Spain. Constantinople was conciliated by Adrian's cooperation in opposing the Iconoclasts in the second Council (787) of Nicaea. Adrian confirmed the council's decrees, but, partly because of faulty translation, they were attacked by Charlemagne. Despite their difference of opinion, the rulers remained in rapport. Charlemagne commemorated Adrian in an epitaph composed by the scholar Alcuin and preserved at St. Peter's in Rome.