Mainz (German: [maɪnts] ^ⓘ; see below) 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.

Mainz is located at the northern end of the Upper Rhine Plain, on the left bank of the Rhine. It is the largest city of Rhenish Hesse, a region of Rhineland-Palatinate that was historically part of Hesse, and is one of Germany's most important wine regions because of its mild climate. Mainz is connected to Frankfurt am Main by the Rhine-Main S-Bahn rapid transit system. Before 1945, Mainz had six boroughs on the other side of the Rhine (see: de:Rechtsrheinische Stadtteile von Mainz). Three have been incorporated into Wiesbaden (see: de:AKK-Konflikt), and three are now independent.

Mainz was founded as Castrum Mogontiacum by Roman general Nero Claudius Drusus in the 1st century BC on the northern frontier of the Roman Empire, and became the capital of the Roman province of Germania Superior. The city was settled by the Franks from 459 on, and in the 8th century it became an important city within the Holy Roman Empire, as capital of the Electorate of Mainz and seat of the Archbishop-Elector of Mainz, the primate of Germany. Mainz Cathedral is one of the three Rhenish Imperial Cathedrals along with Speyer Cathedral and Worms Cathedral. Since the 12th century, Mainz was one of the ShUM-cities [de]—a league formed by the cities of Speyer, Worms and Mainz—which are referred to as the cradle of Ashkenazi Jewish life and as the center of Jewish life during medieval times. The Jewish heritage of these cities is one of a kind, and has been declared the UNESCO World Heritage Site of "ShUM Sites of Speyer, Worms and Mainz" [de]. Mainz is the birthplace of Johannes Gutenberg, who invented the printing press and introduced letterpress printing to Europe, starting the global spread of the printing press. Mainz was heavily damaged in World War II; more than 30 air raids destroyed around half of the old town in the city centre, but many buildings were rebuilt post-war.

Like most cities in the Rhineland, Mainz holds extensive carnival celebrations, that are known as the second-most important in Germany, after the celebrations in Cologne. The borough of Lerchenberg is the seat of ZDF (Zweites Deutsches Fernsehen, lit. "Second German Television"), the second-most important German public service television broadcaster, as well as of 3sat, another television broadcaster, that is jointly operated by public broadcasters from Germany (ARD and ZDF), Austria (ORF), and Switzerland (SRG SSR).

Although the city is situated opposite the mouth of the Main, the name of Mainz is not from Main, the similarity being perhaps reinforced by folk-etymological reanalysis. Main is from Latin Moenis (also Moenus or Menus), the name the Romans used for the river. Linguistic analysis of the many forms that the name "Mainz" has taken on make it clear that it is a simplification of Mogontiacum. The name appears to be Celtic, however, it had also become Roman and was selected by them with a special significance. The Roman soldiers defending Gallia had adopted the Gallic god Mogons (Mogounus, Moguns, Mogonino), for the meaning of which etymology offers two basic options: "the great one", similar to Latin magnus, which was used in aggrandizing names such as Alexander magnus, "Alexander the Great" and Pompeius magnus, "Pompey the Great", or the god of "might" personified as it appears in young servitors of any type whether of noble or ignoble birth.

Mainz has a number of different names in other languages and dialects. In Latin it is known as Mogontiacum (pronounced [mɔɡɔnˈti.akũː]) or Moguntiacum and, in the local Hessian dialect, it is Määnz [mɛːnt͡s] or Meenz [meːnt͡s]. It is known as Mayence [majɑ̃s] in French, Magonza [maˈɡontsa] in Italian, Maguncia [maˈɣunθja] in Spanish, Mogúncia [muˈɣũsjɐ] in Portuguese, Moguncja [mɔˈɡunt͡sja] in Polish, Magentza (מגנצא) in Yiddish, and Mohuč in Czech and Slovak (Czech pronunciation: [ˈmoɦutʃ]).

Before the 20th century, Mainz was commonly known in the Anglosphere either as Mentz, its English version, or by its French version Mayence. It is the namesake of two American cities named Mentz.

Mainz is on the 50th latitude north, on the left bank of the Rhine. The east of the city is opposite where the Main falls into it. As of 2021^[update], the population was 217,272. The city is part of the FrankfurtRheinMain area of 5.9 million people. Mainz can easily be reached from Frankfurt International Airport in 30 minutes by commuter railway or regional trains RE 2 RE 3 RB 31. The river port of Mainz is located on the Rhine and thus on one of the most important waterways in Germany. The container port hub is north of the town centre.