The Lower Danube Circle or Under Danube Circle[1] (German: Unterdonaukreis) was one of the administrative districts (German: Bezirke or Regierungsbezirke) of the Kingdom of Bavaria between 1806 and 1837.

It was named after its main river, the Danube and was the predecessor of the administrative district of Lower Bavaria (Regierungsbezirk Niederbayern).

Its administrative headquarters were in Passau, but the appellate court was in Straubing.

The Unterdonaukreis was initially composed of 10 rural divisions and, since 1809, of the independent city of Passau.

In another territorial reorganization initiated by King Ludwig I on 29 November 1837, the Unterdonaukreis again enlarged and renamed Lower Bavaria (Niederbayern).