1. Fußball- und Sportverein Mainz 05 e. V., usually shortened to 1. FSV Mainz 05, Mainz 05 ([ˌmaɪnts nʊl ˈfʏnf] (listen)) or simply Mainz ([maɪnts] (listen)), is a German sports club, founded in 1905 and based in Mainz, Rhineland-Palatinate. 1. FSV Mainz 05 play in the Bundesliga, the top tier of the German football league system, having most recently been promoted ahead of the 2009–10 season. The club's main local rivals are Eintracht Frankfurt and 1. FC Kaiserslautern. In addition to the football division, 1. FSV Mainz 05 have handball and table tennis departments.

A failed attempt to start a football club in the city in 1903 was followed up two years later by the successful creation of 1. Mainzer Fussballclub Hassia 1905. After a number of years of play in the Süddeutschen Fußballverband (South German Football League), the club merged with FC Hermannia 07 – the former football side of Mainzer TV 1817 – to form 1. Mainzer Fussballverein Hassia 05, which dropped "Hassia" from its name in August 1912. Another merger after World War I, in 1919, with Sportverein 1908 Mainz, resulted in the formation of 1. Mainzer Fußball- und Sportverein 05. Die Nullfünfer ("05") was a solid club that earned several regional league championships in the period between the wars and qualified for the opening round of the national championships in 1921, after winning the Kreisliga Hessen.

In the late 1920s and early 1930s, the club earned decent results in the Bezirksliga Main-Hessen – Gruppe Hessen, including first-place finishes in 1932 and 1933. This merited the team a place in the Gauliga Südwest, one of 16 new first-division leagues formed in the re-organization of German football under the Third Reich. The club only managed a single season at that level before being relegated, due to the high intensity play that they were unable to keep up with. Karl Scherm scored in 23 out of 44 matches with Mainz during his last season. In 1938, Mainz was forced into a merger with Reichsbahn SV Mainz and played as Reichsbahn SV Mainz 05 until the end of World War II.