The Duchy of Lower Lorraine or Lower Lotharingia (also referred to sometimes as Lothier or Lottier in titles), established in 959 was a stem duchy of the medieval German kingdom, which encompassed 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 was 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.In 959, King Henry's son Duke Bruno the Great divides Lotharingia into two duchies: Lower and Upper Lorraine (or Lower and Upper Lotharingia) and grants Count Godfrey I of Mons (Hainaut) the title of a Duke of Lower Lorraine.

Godfrey's lands are to the north (lower down the Rhine river system), while Upper Lorraine is to the south (further up the river system).

Both duchies form the western part of the Holy Roman Empire established by Bruno's elder brother Emperor Otto I in 962.Both Lotharingian duchies take very separate paths thereafter: Upon the death of Godfrey's son Duke Richar, Lower Lorraine is directly ruled by the Emperor, until in 977 Otto II enfeoffs Charles, the exiled younger brother of King Lothair of France.

Lower and Upper Lorraine are once again briefly reunited under Gothelo I from 1033 to 1044.

After this, the Lower duchy is quickly marginalized, while Upper Lorraine comes to be known as simply the Duchy of Lorraine.Over the next decades the significance of the Duchy of Lower Lorraine diminishes and furthermore is affected by the conflict between Emperor Henry IV and his son Henry V: In 1100 Henry IV had enfeoffed Count Henry of Limburg, who Henry V, having enforced the abdication of his father, immediately deposes and replaces by Count Godfrey of Louvain.

Upon the death of Duke Godfrey III in 1190, his son Duke Henry I of Brabant inherits the ducal title by order of Emperor Henry VI at the Diet of Schwäbisch Hall.

Thereby the Duchy of Lower Lorraine finally loses its territorial authority, while the remnant Imperial fief held by the Dukes of Brabant is later called the Duchy of Lothier (or Lothryk).