In the East German era, 1. FC Union Berlin was known for its competition with BFC Dynamo and was thought to be associated to the powerful state security services that was part of East Germany (Stasi). Union was backed by the district leaders of the ruling SED party, and was backed by local state-owned businesses. The club played a significant role in the opposition that was not officially against the ruling authorities of the communist regime. Between 1979 between 1979 and 1989, BFC Dynamo was awarded a string of ten East German league titles, with a plethora of allegations of sports infractions fueling the rivalry. Likewise, tensions between both sets supporters took place. BFC Dynamo was seen as the ultimate representative of the security services, having advantages in the selection of players, financial backing and the influence associated with Erich Mielke. The supporters of Union created an image for their team as an forever underdog and was embedded within the class of workers. Union became the most well-known football club of East Berlin.
It is reported that fans from 1. FC Union Berlin often chanted "The wall must come down! " in allusion in reference to the Berlin Wall, when the players were able to create an obstacle when they played free kicks during the the 1980s. But some sources claim that this is an oversimplified myth. The supporters of Union considered themselves to be stubborn and rebellious. This image must not be misinterpreted as resistance in the real sense. For some people who are supporters of Union the reputation of dissidents is a mythology that was created following Die Wende. The honorary the president of Union Gunter Mielis stated: "Union was not a organization of rebels However, we had to face the economic and political opposition repeatedly. We were able to rely on our members". Political issues were not the forefront. The majority of the people who voted for Union were ordinary football fans. There was no political group at Union. One fan of Union in during the East German era has said: "With the best of intentions, Union fans did not help in the dismantling of GDR. There was no way we were attracted by football. There's a cliche that says the club being for the people who were enemies of the state but we weren't one of them". The supporters from Union during time to East German era have testified that Union had the highest importance. It was also said that the association with Union was in part with Kopenick.
In spite of 1. FC Union Berlin and Hertha BSC being the two largest clubs in Berlin the rivalry between the two clubs has been much less evident. The bonds of friendship between both clubs emerged within the dividing walls of Berlin. The first contact between the supporters of both clubs began during the 70s. Fans of Hertha went to Hertha's Stadion An der Alten Forsterei and those of Union joined the supporters of Hertha during times when Hertha was playing in Eastern Bloc countries, such as the quarter-finals of the 1978-1979 UEFA Cup against Dukla Prague. The slogans and chants included "Ha-Ho-He There is only two teams in the Spree - Union and Hertha BSC" (German: Ha-Ho-He, there are only two Mannschaften an der Spree - Union und Hertha BSC) and "Hertha and Union - one nation" (German: Hertha und Union - A Nation) was a hit with the two sets of supporters. The two sets of supporters came together for the first time after the opening of the Berlin wall during the first edition of the indoor tournament "Internationales Berliner Hallenfussballturnier" in the Werner-Seelenbinder-Halle on 18-20 January 1990. The supporters from Union and Hertha have since also sung the xenophobic and nationalist songs.
On January 27, 1990, just 79 days after that fall of the Berlin Wall, Hertha hosted Union Berlin at the Olympiastadion in a friendly match in front of 52,000 people. Fans from both clubs paid for their tickets in East and West Germany's currencies, and sung songs about German reunion as Hertha defeated Union Berlin by a score of 2-1. Two decades after, on the 17th of September 2010, the two teams played each other for the second time, for their first match in competition on the Stadion An der Alten Forsterei and drew 1-1 in the 2. Bundesliga. 2 November 2019: Union Berlin faced Hertha at the Stadion An der Alten Forsterei in the first match between the two clubs of the top division in German football. A 87th minute Sebastian Polter penalty secured a 1-1 win for Union in a match that was briefly stopped by the referee Deniz Aytekin following the incident of fireworks shot by Hertha fans that landed among Union Berlin fans, as also on the field. 1100 police officers were on duty during the game as Hertha supporters torching Union Berlin shirts, flags and scarves in the course of the match. The Hertha supporters Hertha were with 20-25 supporters from BFC Dynamo in the guest block. At the end of full time, Union Berlin goalkeeper Rafal Gikiewicz received praise from media and fans alike for escorting Union Berlin ultras from the field of play after a minor pitch invading designed to target Hertha supporters.
Union Berlin also holds rivalries with Hansa Rostock, Dynamo Dresden and Magdeburg that date back to the time when the teams played with each other in the DDR Oberliga.
Recently the club has also developed an adversary against RB Leipzig, following the transfer of teams and licenses from the fifth division team SSV Markranstadt financed by Red Bull GmbH and the move of Leipzig in Bundesliga. Bundesliga system. In 2011 Union Berlin ran adverts against the club's investment and also cancelled an official pre-season match against the team. On the 21st September 2014 Union Berlin fans staged an uninvolved protest during the first 15 minutes of a two. Bundesliga home match with RB Leipzig, labelling RB Leipzig as a "marketing product that is pushed through financial interest" with "brainwashed fans in the seats". Union Berlin won the game 1-1. On the 18th of August the first ever Bundesliga match, played in the home stadium in a match against RB Leipzig, the club's oldest ultras group known as the Wuhlesyndikat was able to successfully call for a 15-minute silence before the team's 2019-20 opening.