Lotharingia was a historical region and an early medieval polity that existed during the late Carolingian and early Ottonian, from the middle of the 9th to the middle of the 10th century. It was established in 855 by the Treaty of Prüm as a distinct kingdom within the Carolingian Empire, but abolished already in 869-870 when it was divided by the Treaty of Meerssen. It was territorially reunited in 880 by the Treaty of Ribemont, and reestablished as a kingdom from 895 to 900. Since 903 it was organized as a duchy, which existed up to 959, when it was divided into two distinct duchies: Upper Lotharingia (southern half), and Lower Lotharingia (northern half). The regional name Lotharingia means, approximately, "the land of Lothair", and was derived from the name of its first ruler, king Lothair II, who received this territory as his share of the Kingdom of Middle Francia. The region comprised present-day Lorraine (France), Luxembourg, parts of modern Germany west of the Rhine, most of Belgium, and the Netherlands.

Lotharingia resulted from the tripartite division in 855 of the kingdom of Middle Francia, which itself was formed after the threefold division of the Carolingian Empire by the Treaty of Verdun of 843. Conflict between East and West Francia over Lotharingia was based on the fact that these were the old Frankish homelands of Austrasia, so possession of them was a matter of great prestige to their kings as true claimant of Frankish imperial legacy.

Lotharingia was known as regnum quondam Lotharii or regnum Lotharii ("kingdom [once] Lothair's").

Its inhabitants were known as Lotharii (from Lotharius), Lotharienses (from Lothariensis), or Lotharingi (which gives the modern Dutch, German, and Luxembourgish names for the province Lotharingen, Lothringen, and Loutrengen respectively). The latter term, formed with the Germanic suffix -ing, indicating ancestral or familial relationships, gave rise to the Latin term Lotharingia (from the Latin suffix -ia, indicating a country) in the 10th century.

Later French terms such as "Lorraine" and "Lothier" are derived from the Latin term.

In 817, Emperor Louis the Pious made plans for division of the Carolingian Empire among his three sons after his death. Unforeseen in 817 was a further heir besides Louis's three grown sons. A fourth son, Charles the Bald, was born to Louis's second wife Judith of Bavaria in 823. When Louis tried in 833 to re-divide the empire for the benefit of Charles, he met with opposition from his adult sons, Lothair, Pepin, and Louis. A decade of civil war and fluctuating alliances followed, punctuated by brief periods of peace.

Pepin died in 838, and Louis the Pious in 840. The remaining three brothers made peace and divided the Empire with the 843 Treaty of Verdun. Lothair, as the eldest, kept the imperial title and received a long strip of territories stretching from the North Sea to southern Italy. The logic of the division was that Lothair had the crown of the Kingdom of Italy, which had been his subkingdom under Louis the Pious, and that as emperor he should rule in Aachen, the capital of the first Carolingian emperor, Charlemagne, and in Rome, the ancient capital of emperors. Middle Francia (Latin Francia media) thus included all the land between Aachen and Rome, and it has sometimes been called by historians the "Lotharingian axis".

In 855, when Lothair I was dying in Prüm Abbey, he divided his kingdom among his three sons with the Treaty of Prüm. To the eldest son, Louis II, went Italy, with the imperial title. To the youngest, Charles, still a minor, went Provence. To the middle son, Lothair II, went the remaining territories to the north of Provence, a kingdom which lacked ethnic or linguistic unity.