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).

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.

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.