Lewandowski’s 40th minute strike from the penalty spot took his season tally to 40 goals in all competitions, including 26 in the Bundesliga for the Bavarians.
The current Bundesliga record is held by German legend Gerd Muller who netted 40 goals in the 1971-72 season, with the Pole having that in his sights.
“Lewy is making a very good impression, he’s top fit,” Bayern boss Hansi Flick said: “He scored another goal.”
Lewandowski was a beneficiary of the timing of the coronavirus suspension, having overcome a knee injury during the lay-off to return upon the league’s resumption this weekend.
The Poland international has scored more goals this season than any other player in Europe’s top five leagues.
His season tally of 40 goals also adds to a remarkable recent record across the past five seasons which has seen him score 42, 43, 41 and 40 times.
Despite Flick’s joy for the his striker, he was more content with his side’s ability to keep a clean sheet against Union Berlin.
“It was a workmanlike but deserved victory,” Flick said.
“We struggled at first, but I must pay my team a compliment on the 90 minutes.
“We could surely be heard shouting, ‘Lads, you need to keep a clean sheet.’ So at the end of the day I’m satisfied with the result.”