Lothair I or Lothar I (Dutch: Lotharius, German: Lothar, French: Lothaire, Italian: Lotario) was the Holy Roman Emperor (817–855, co-ruling with his father until 840), and the King of Bavaria (815–817), 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 Pippin 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.
In 855 he became seriously ill, and despairing of recovery renounced the throne, divided his lands between his three sons, and on the 23rd of September entered the monastery of Prüm, where he died six days later. He was buried at Prüm, where his remains were found in 1860.
In 870, Lothair's kingdom was divided between his three sons in a deal called the Treaty of Mersen: the eldest, Louis II, received Italy and the title of emperor; the second, Lothair II, received Lotharingia; the youngest, Charles, received Provence.