Lotharingia (Latin: regnum Lotharii, regnum Lothariense, Lotharingia; French: Lotharingie; German: Reich des Lothar, Lotharingien, Mittelreich; Dutch: Lotharingen) was a short-lived medieval successor kingdom of the Carolingian Empire. As a more durable later duchy of the Ottonian Empire, it comprised present-day Lorraine (France), Luxembourg, Saarland (Germany), Netherlands, and the eastern half of Belgium, along with parts of today's North Rhine-Westphalia (Germany), Rhineland-Palatinate (Germany) and Nord (France). It was named after King Lothair II, who received this territory after his father Lothair I's kingdom of Middle Francia was divided among his three sons in 855.

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 as true claimant of Frankish imperial legacy.