Mainz-Kastel is a district of the city Wiesbaden, which is the capital of the German state Hesse in western Germany.
Kastel is the historical bridgehead of Mainz, the capital of the German state Rhineland-Palatinate and is located on the right side of the Rhine river. Kastel faces the historical center of Mainz and the two cities are connected by a road bridge. Kastel is located about one kilometer below today's water mouth of the river Main into the Rhine.
In its long history Kastel repeatedly belonged to Mainz and was formally incorporated on 1 April 1908. Since Mainz was part of the French occupation zone (formed after World War II) and Kastel was part of the American occupation zone, the Americans ordained that Kastel should be part of the administration of Wiesbaden. Ever since 25 July 1945, Kastel belongs to the Hessian state capital Wiesbaden.