Mainz is the capital and largest city of the German state of Rhineland-Palatinate, and with around 223,000 inhabitants, it is Germany's 35t…Mainz is the capital and largest city of the German state of Rhineland-Palatinate, and with around 223,000 inhabitants, it is Germany's 35th-largest city. It lies in the Rhine-Main Metropolitan Region—Germany's second-largest metropolitan region after Rhine-Ruhr—which also encompasses the cities of Frankfurt am Main, Wiesbaden, Darmstadt, Offenbach am Main, and Hanau.