| Native Name: | Unterdonaukreis |
| Common Name: | Lower Danube Circle |
| Subdivision: | Regierungsbezirk |
| Nation: | the Kingdom of Bavaria |
| Capital: | Passau |
| Year Start: | 1806 |
| Year End: | 1837 |
| Area Lost1: | Innviertel |
| Lost To1: | Austrian Empire |
| Area Lost Year1: | 1816 |
| Area Gained1: | Innviertel |
| Gained From1: | Austrian Empire |
| Area Gained Year1: | 1810 |
| Image Map Caption: | Districts of Bavaria, including Tyrol, in 1808 |
| S1: | Lower Bavaria |
| Flag S1: | Flag of Lower Bavaria.svg |
| S2: | Upper Bavaria |
| Flag S2: | Flagge Oberbayern.svg |
| S3: | Upper Palatinate |
| Flag S3: | Flagge Oberpfalz.svg |
| S4: | Upper Austria |
| Flag S4: | Flag of Tirol and Upper Austria.svg |
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 district was divided in the following judicial districts (Landgerichte), according to the original borders of the districts of the former territories (Herrschaftsgerichte):
In the years between 1806 and 1808 the Kingdom of Bavaria was divided into 15 'circles'. Their names were taken from their main rivers. The Unterdonaukreis was initially composed of 10 rural divisions and, since 1809, of the independent city of Passau. In 1810, the Unterdonaukreis was enlarged. In another territorial reorganization initiated by King Ludwig I on 29 November 1837, the Unterdonaukreis again enlarged and renamed Lower Bavaria (Niederbayern). District headquarters were moved from Passau to Landshut.