1. Fußballclub Union Berlin e. V., commonly known as 1. FC Union Berlin (German pronunciation: [ˈeːɐ̯stɐ ʔɛfˈtseː ʔuˈni̯oːn bɛʁˈliːn]) or Union Berlin, is a professional German football club in Köpenick, Berlin. The club’s origins can be traced to 1906, when its predecessor FC Olympia Oberschöneweide was founded. From 2009 until 2019, they competed in the 2. Bundesliga, the second tier of German football. In 2019, after defeating VfB Stuttgart in the relegation play-offs, Union won promotion to the Bundesliga top flight for the first time in the club’s history, for the 2019–20 season.

During the Cold War, Union was based in East Berlin, joining the German league structure upon the reunification of the city and country in 1990.

Their home ground Stadion An der Alten Försterei is the second-largest in the German capital. It has been home to Union Berlin and its forerunners since it opened in 1920. The stadium also hosts concerts and the annual Weihnachtssingen Christmas carols event.

The club has become well known for its enthusiastic and creative fan base and its chant “Eisern Union” (Iron Union).