(late 827–25 Jan. 844)
A Roman of aristocratic family, cardinal priest of S. Marco, he owed his election to the votes of the lay nobility. In compliance with Lothair I's constitution of 824, his consecration (29 Mar. 828) was deferred until an imperial legate had approved his election and he himself had sworn allegiance. The dependence of the papal state on the Holy Roman emperor remained effective in the early years of his reign; when in 829 imperial judges sitting in Rome decided against the Roman church in favour of the abbey of Farfa, which Lothair had exempted from tribute to the holy see in Paschal I's time, the Frankish court seems to have rejected his appeal.
This dependence was loosened as a result of the dynastic struggles between Emperor Louis I the Pious (814–40) and his sons by his first marriage Lothair I (840–55), Pepin (d. 838), and Louis the German (d. 876). When these three rebelled against their father, Gregory supported Lothair and accompanied him across the Alps to Francia. He hoped that his mediation would be understood as intended to promote peace, but the mass of Frankish bishops were outraged by his partisanship, reminded him of his oath of fealty to Louis, and threatened excommunication if he persisted in disloyalty. He was at first shaken but, encouraged by leading churchmen like Agobard of Lyons (769–840) and Wala of Corbie (c. 755–836), furiously rebuked his critics, insisting that the authority of St Peter's successor was supreme, that the peace and unity of the empire were his concern, and that the papacy, entrusted with the care of men's souls, was superior to the imperial power. When the armies faced each other at Rotfield, near Colmar, in summer 833, the brothers persuaded him to go to Louis's camp to negotiate, but when he returned with what seemed a reasonable basis for a reconciliation, he found that he had been duped by Lothair. On the night of the pope's return most of Louis's supporters deserted him, and on 30 June he had to surrender unconditionally, only to be deposed and humiliated. Gregory returned to Rome from ‘the field of lies’ bitterly regretting his intervention. Louis was restored in Mar. 834, and in 837 reopened relations with the pope, with the object ostensibly of making a pilgrimage to Rome but really to detach him from Lothair. Gregory was delighted and sent an embassy to Louis, but it was held up by Lothair at Bologna; he managed, however, to smuggle a letter through. After Louis's death (20 June 840), he made timid attempts to mediate in the bloody conflict which ensued between the brothers, but without success.
Apart from these ineffective political manoeuvres, little is known of Gregory's pontificate. The Saracens, established in Sicily since 827, were now a constant threat to mainland Italy: they had twice, in 813 and 827, attacked Centumcellae (the modern Civitavecchia); to counter this he built a powerful fortress, named Gregoriopolis, at Ostia. In 831/2 he received Anskar, since 826 missionary in Denmark and recently consecrated bishop of Hamburg, gave him the pallium, and named him legate for Scandinavia and the Slav missions. In 831 he also received the liturgist Amalarius of Metz (c. 780–c. 850) and assigned him an archdeacon to teach him Roman liturgical usage. At Gregory's suggestion, four years later, Louis I extended the observance of All Saints' Day throughout his dominions. In Rome the pope spent lavishly on building and decorating churches; his portrait in mosaic, commissioned by himself, can be seen in the apse of S. Marco. Among his other works was the reconstruction of a ruined aqueduct linked with the Janiculum, which provided not only water for domestic purposes but power to operate mills.