Unterelsaß (or Unterelsass) (Lower Alsace, French: Basse-Alsace) was the name for a district ("Bezirk") in the northern part of Elsaß-Lothringen (Alsace-Lorraine) when it was part of the German Empire from 1871 to 1918.
The capital of the district was Straßburg. It comprised the subdistricts ("Kreise") of
The district of Unterelsaß corresponds exactly to the current département of Bas-Rhin.
The flag is a white bar in a red field decorated on each side with a white lace motif. The union of this flag with that of Oberelsaß forms the flag of modern Alsace.