Frankish emperor, whose attempt to gain sole rule over the Frankish territories was checked by his brothers.

The eldest son of the emperor Louis I the Pious and a grandson of Charlemagne, Lothar was made king in Bavaria after Louis succeeded Charlemagne in 814, and in 817 he was made joint emperor. Under the Ordinatio imperii, a decree issued by Louis in 817 to provide for the unity of the empire after his death, Lothar's younger brothers, Pippin (Pippin I) and Louis (Louis II) (later called the German), were to receive their own kingdoms, Aquitaine and Bavaria, but were to remain under the general suzerainty of Lothar.

Ruler in Italy from 822, Lothar was crowned emperor by Pope Paschal I (Paschal (I)) in 823. He issued the Constitutio Romana (824), affirming imperial sovereignty over Rome and demanding an oath of fealty from the pope. When in 829 Louis I, under the influence of his second wife, Judith, revised the Ordinatio imperii to grant part of the empire previously granted to Lothar to his son by Judith, Charles (Charles II) (later called the Bald), Lothar broke with the imperial government. A palace revolution forced his reappointment as coemperor in 830, but he was again deposed shortly afterward.

In 833 discontent with the rule of Louis I the Pious ended in a revolt of the three elder sons, led by Lothar, and Lothar replaced the deposed Louis. Louis was restored to power the following year, however, and Lothar's rule was restricted to Italy.

When Pippin died in December 838, Louis I drew up a new partition scheme, dividing the empire, aside from Bavaria and neighbouring areas, which were left to Louis the German, between Lothar and Charles II the Bald, with Lothar taking the eastern portion. Lothar was to have the title of emperor but without the suzerainty over the other princes that had been granted by the Ordinatio imperii of 817.

On Louis I's death (840), Lothar again claimed his rights under the Ordinatio of 817, but his brothers, Louis and Charles, defeated him at the Battle of Fontenoy (841). The Treaty of Verdun (Verdun, Treaty of) (August 843) left Lothar the Middle Realm of the Frankish dominions, from the North Sea to Italy, while Louis received the eastern and Charles the western territory. The imperial title fell to Lothar.

After granting the government of Italy to his eldest son, Louis II, as early as 844, Lothar partitioned his realm between Louis (emperor from 850) and his two other sons, Lothar (Lothar (II)) and Charles, in 855. Then he abdicated and became a monk.