The Untermainkreis (German: Lower Main District) was one of the administrative districts (German: Bezirke or Regierungsbezirke) of the Kingdom of Bavaria between 1817 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 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.