Lothair I or Lothar I (Dutch and Medieval Latin: Lotharius; German: Lothar; French: Lothaire; Italian: Lotario) (795 – 29 September 855) was emperor (817–855, co-ruling with his father until 840), and the governor of Bavaria (815–817), King of Italy (818–855) and Middle Francia (840–855).

Lothair was the eldest son of the Carolingian emperor Louis the Pious and his wife Ermengarde of Hesbaye, daughter of Ingerman the duke of Hesbaye. On several occasions, Lothair led his full-brothers Pepin I of Aquitaine and Louis the German in revolt against their father to protest against attempts to make their half-brother Charles the Bald a co-heir to the Frankish domains. Upon the father's death, Charles and Louis joined forces against Lothair in a three-year civil war (840–843). The struggles between the brothers led directly to the breakup of the Frankish Empire assembled by their grandfather Charlemagne, and laid the foundation for the development of modern France and Germany.