| Native Name: | Untermainkreis |
| Common Name: | Lower Main Circle |
| Subdivision: | Regierungsbezirk |
| Nation: | the Kingdom of Bavaria |
| Capital: | Würzburg |
| Year Start: | 1817 |
| Year End: | 1837 |
| Title Leader: | Generalkommissär |
| Leader1: | Max Freiherr von Lerchenfeld |
| Year Leader1: | 1814–1817 |
| Leader2: | Franz Wilhelm Freiherr von Asbeck |
| Year Leader2: | 1817–1825 |
| Leader3: | Max Freiherr von Zu Rhein |
| Year Leader3: | 1826–1832 |
| Leader4: | Karl Freiherr von Stengel |
| Year Leader4: | 1832 |
| Leader5: | Graf August von Rechberg und Rothenlöwen |
| Year Leader5: | 1833–1837 |
| Leader6: | Ferdinand von Andrian-Werburg |
| Year Leader6: | 1837 |
| Image Map Caption: | Districts of Bavaria, including Tyrol, in 1808 |
| P1: | Grand Duchy of Würzburg |
| Flag P1: | Flag of the Grandduchy of Wurzburg.svg |
| P2: | Principality of Aschaffenburg |
| Flag P2: | Banner of the Electorate of Mainz.svg |
| P3: | Grand Duchy of Hesse |
| Flag P3: | Flagge Großherzogtum Hessen ohne Wappen.svg |
| P4: | Grand Duchy of Baden |
| Flag P4: | Flagge Großherzogtum Baden (1891–1918).svg |
| S1: | Lower Franconia |
| Flag S1: | Flag of Lower Franconia.svg |
The Untermainkreis (German: Lower Main District) was one of the administrative districts (German: Bezirke or Regierungsbezirke) of the Kingdom of Bavaria between 1806 and 1837 named after its main river Main. It was the predecessor of the administrative district of Lower Franconia (Regierungsbezirk Unterfranken). Administrative headquarters were in Würzburg.
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 districts. Their names were taken from their main rivers. The Untermainkreis was only established in 1817 after the Grand Duchy of Würzburg, the Principality of Aschaffenburg and other territories had been annexed to Bavaria. Some of the judicial districts had already been established in 1814 and 1816.In the process of another territorial reorganization initiated by King Ludwig I on 29 November 1837, the Untermainkreis was renamed Lower Franconia and Aschaffenburg (Unterfranken und Aschaffenburg). The town name of Aschaffenburg was dropped in 1946.