1. Fußballclub Union Berlin e. V., commonly known as 1. FC Union Berlin ([ˈeːɐ̯stɐ ʔɛfˈt͡seː ʔuˈni̯oːn bɛʁˈliːn] (listen)) 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. 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. From 2009 until 2019, they competed in the 2. Bundesliga, the second tier of German football. In 2019, Union won promotion to the Bundesliga for the first time in the club's history. In 2022, the club qualified for the UEFA Europa League.

The home ground of the club is the Stadion An der Alten Försterei. It is the second-largest in the German capital and 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.

As of 2022, Union Berlin has 45,000 official members. The club has become well known for its enthusiastic and creative fan base and its chant "Eisern Union" (Iron Union).