| Native Name: | Mainkreis (until 1817) Obermainkreis (from 1817) |
| Common Name: | Main Circle |
| Subdivision: | Regierungsbezirk |
| Nation: | the Kingdom of Bavaria |
| Capital: | Bamberg (1806–1812) Bayreuth (1812–1837) |
| Year Start: | 1806 |
| Year End: | 1837 |
| Title Leader: | Generalkommissär |
| Leader1: | Friedrich Karl von Thürheim |
| Year Leader1: | 1810–1814 |
| Leader2: | Constantin Ludwig Freiherr von Welden |
| Year Leader2: | 1815–1832 |
| Leader3: | Ferdinand Freiherr von Andrian-Werburg |
| Year Leader3: | 1832–1837 |
| Image Map Caption: | Districts of Bavaria, including Tyrol, in 1808 |
| S1: | Upper Franconia |
| Flag S1: | Flagge Oberfranken.svg |
| S2: | Upper Palatinate |
| Flag S2: | Flagge Oberpfalz.svg |
The Mainkreis (German: [River] Main Circle) was one of the 15 administrative districts (German: Regierungsbezirke) of the Kingdom of Bavaria between 1806 and 1837. The district was named after its main river Main and renamed Obermainkreis (Upper Main Circle) in 1817. It was the predecessor of the Regierungsbezirk Oberfranken (Administrative Regional District of Upper Franconia).
The district was divided in the following judicial districts (Landgerichte = LG), according to the original borders of the districts of the former territories (Herrschaftsgerichte = HG):
In the years between 1806 and 1808 the Kingdom of Bavaria was divided in 15 (state) districts, whose names were taken from their rivers. The Mainkreis, with Bamberg as its capital, was initially composed of 18 rural divisions and, since 1809, of the independent cities of Bamberg and Schweinfurt. In 1810 it grew significantly with the annexation of the Naabkreis. It was again enlarged when it received 12 rural divisions from the former Principality of Bayreuth, whereupon the district administration was moved to Bayreuth. In 1817, the Mainkreis was renamed Obermainkreis (District of the Upper Main River) and slightly expanded. At the same time, a new district with the name of Untermainkreis (District of the Lower Main) was created downriver. The Territorial Reorganization (Gebietsreform) of 29 November 1837, implemented at the request of King Ludwig I, split the judicial districts of the former Naabkreis from the Obermainkreis, assigning them to Oberpfalz (Upper Palatinate) instead, and renamed the shrunken Obermainkreis “Oberfranken” (Upper Franconia).