Mainz-Kostheim is a district administered by the city of Wiesbaden, Germany. Mainz-Kostheim was formerly a district of the city of Mainz, until the public administration by the city of Wiesbaden was decided on August 10, 1945. The reason for this had been the easy control of the Allied Occupation Zones in Germany, where the Rhine formed the border between the American sector and the French sector.