Marius Bülter scored twice for the promoted side and Sebastian Andersson grabbed the other goal as Union won at the third attempt following a 1-1 draw in Augsburg and 4-0 loss at home to Leipzig.
Dortmund had opened its league campaign by scoring eight goals in two games but wasn’t able to cope with the hosts’ pressing game.
Union, which clinched promotion via a playoff in May, is the first Bundesliga team to have played in East Germany’s Oberliga since Energie Cottbus was relegated in 2009. The Oberliga was disbanded in 1991 following German reunification the year before.
Earlier, Alphonso Davies scored his first goal of the season as Bayern Munich bounced back to destroy Mainz.
Bayern coach Niko Kovac handed first starts to Philippe Coutinho and Ivan Perisic, but it was their substitutes, Thomas Müller and Davies respectively, who combined for the final goal in the 81st minute when Müller gave the Canadian teenager a tap-in.
Robert Lewandowski also scored to take his tally to six goals in three Bundesliga games this season, with Benjamin Pavard, David Alaba, Perisic and Kingsley Coman grabbing the other goals after Jean-Paul Boetius‘ early opener for Mainz.
Coutinho played just behind Lewandowski and was involved early as he won the ball to set up the Poland striker for the game’s first big chance in the fourth minute.
But Boetius stunned home fans two minutes later, when Pavard left him free to head Ronael Pierre-Gabriel’s cross past the static Manuel Neuer in the Bayern goal.
Bayern seemed out of sorts with poor passing. But the home side gradually gained the upper hand and Pavard equalized off Perisic’s cross in the 36th.
Alaba made it 2-1 with a brilliant free kick just before the break, and there was no stopping the home side after that.
Also Saturday, Hertha Berlin conceded two own-goals in a 3-0 defeat at Schalke and Cologne rallied from a goal down to win 2-1 in Freiburg for its first victory back in the top flight. Paderborn drew at Wolfsburg 1-1, and Hoffenheim held Bayer Leverkusen 0-0.
Source: FOX Sport