Kelsterbach is a town in Groß-Gerau district in Hessen, Germany and part of the Frankfurt Rhein-Main urban area. It is located on Frankfurt's southwestern outskirts at a bend on the left bank of the river Main, right where a small brook, called the Kelster joins the river.

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.