Kleinostheim is a community in the Aschaffenburg district in the Regierungsbezirk of Lower Franconia (Unterfranken) in Bavaria, Germany.

The Main is the longest tributary of the Rhine. It rises as the White Main in the Fichtel Mountains of northeastern Bavaria and flows west through central Germany for 525 kilometres (326 mi) to meet the Rhine below Rüsselsheim, Hesse. The cities of Mainz and Wiesbaden are close to the confluence.