The Duchy of Lower Lorraine or Lower Lotharingia (also referred to sometimes as Lothier or Lottier in titles), established in 959 is a stem duchy of the medieval German kingdom, which encompasses part of modern-day Belgium, the Netherlands, the northern part of the German Rhineland and a part of northern France in the present Nord-Pas-de-Calais .It is created out of the former Middle Frankish realm of Lotharingia under King Lothair II, that had been established in 855.

Lotharingia is divided for much of the later ninth century, reunited under Louis the Younger by the 880 Treaty of Ribemont and upon the death of East Frankish king Louis the Child in 911 it joins West Francia under King Charles the Simple.

It then forms a duchy in its own right, and about 925 Duke Gilbert declares homage to the German king Henry the Fowler, an act which King Rudolph of France is helpless to revert.

From this time on Lotharingia (or Lorraine) remains a German stem duchy, the border with France does not change throughout the Middle Ages.