The Mainkreis (German: [River] Main Circle) was one of the 15 administrative districts (German: Regierungsbezirke) of the Kingdom of Bavaria between 1806 and 1837.

It was the predecessor of the Regierungsbezirk Oberfranken (Administrative Regional District of Upper Franconia).

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 1817, the Mainkreis was renamed Obermainkreis (District of the Upper Main River) and slightly expanded.

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).