‘Stadtmitte’ means ‘city centre’, though the station is not at Berlin's centre.
The U2 platform opened in 1908 as Friedrichstraße, the U6 platform in 1923 as Leipziger Straße. In 1924 the whole station was renamed Friedrichstadt, and in 1936 Stadtmitte. It lies neither at Berlin’s centre nor at its geographical midpoint: according to the BVG, the name was chosen because other names were already taken or would cause confusion. The long passage between the platforms is known as the ‘Mäusetunnel’ (mouse tunnel).