Becker keeps Union on Champions League course at Gladbach
Becker's ninth league goal of the season, a volley after a looping cross from Jerome Roussillon, snared all three points for the visitors, sending them into third place.
Union manager Urs Fischer praised his side's "disciplined performance" saying "there wasn't a phase in the game where we eased up".
"When the whistle went, we were the deserved winners."
Freiburg's earlier win over Schalke meant the Black Forest side had overtaken Union into third before kickoff.
With RB Leipzig and Bayer Leverkusen also hot on their trail, Union endured a poor first half before upping the intensity in the second, Becker slamming in after 60 minutes.
With 31 goals against, Union have conceded the fewest of any side in the Bundesliga this season.
Union, who are only in their fourth year in the top division, are now on course for a remarkable maiden Champions League appearance.
Freiburg fourth
A first-half brace from Michael Gregoritsch helped lift Freiburg up to fourth after a 4-0 home victory against struggling Schalke.
Like Union, Freiburg have never played in the Champions League.
"The team knew what was coming for us and we were ready. We were determined and we deserved to win," said Freiburg coach Christian Streich.
His Schalke counterpart Thomas Reis admitted his team could "be thankful that we didn't lose by more".
"We were in no way an equal opponent. There's nothing much more I can say about that today," he said.
Gregoritsch scored after just seven minutes, tapping home after a perfectly-weighted Ritsu Doan assist.
The Austrian used his head to double Freiburg's lead late in the first half, glancing in a cross from defender Christian Gunter.
Doan was again the creative force early in the second half for Freiburg's third, pushing past several defenders to find Gregoritsch in the box.
He then threaded the ball through for Lucas Holer, who blasted the ball at Schalke goalkeeper Alexander Schwolow.
Schwolow failed to properly grasp the effort, allowing the ball to trickle into the goal for Freiburg's third.
Freiburg defender Matthias Ginter added a fourth with eight minutes remaining.
Schalke, who went eight games unbeaten early in 2023, have now lost three of their past four.
The Royal Blues, a traditional German football giant, are in grave danger of being relegated straight back to the second division.
They sit second from bottom and three points from safety, with five rounds of the season remaining.
Bayer Leverkusen kept their strong run of form under manager Xabi Alonso alive, winning 2-0 at home over 10-man RB Leipzig.
Leverkusen, who on Thursday made it through to the semi finals of the Europa League with a win at Union Saint-Gilloise, are in sixth place, five points outside of the top four.
They took the lead late in the first half through Adam Hlozek.
France winger Moussa Diaby burst through the penalty area, before cutting back to the unmarked Hlozek inside the box. The Czech Republic striker hit a left-footed finish past Janis Blaswich.
With Leipzig looking to salvage something from the encounter late in the game, Hungary midfielder Dominik Szoboszlai brought down Jeremie Frimpong in the box, picking up a second yellow.
Nadiem Amiri converted from the spot, sealing a comfortable win for Leverkusen.
Leverkusen have now gone unbeaten in their past 13 matches in all competitions, a run of 10 victories and three draws.
Alonso, who took over with the side in danger of relegation, said Leverkusen won because "we were ready to fight and to suffer".
Leipzig's loss leaves them two points outside the Champions League spots.
