Former Germany international Kruse was targeted after he was substituted in the 83rd minute of Union's 2-1 win at Mainz in early October.
He had beer cups thrown over him as he walked behind the goal on his way off the pitch, resulting in a soaking for the 33-year-old.
Kruse kept his cool and said in a post-match interview: "Why should something like that provoke me? What should I say to such nonsense heads?"
Mainz was charged over the behaviour of its fans and German Football Association (DFB) chiefs have now passed judgement.
"The sports court of the DFB fined the Bundesliga club Mainz 8,000 euros in single judge proceedings after the DFB control committee brought charges for unsportsmanlike conduct on the part of its supporters," the DFB said in a statement.
"In the 83rd minute of the Bundesliga game against Union Berlin on October 3, 2021, the Berlin player Max Kruse was showered with beer by spectators from Mainz after his substitution. The association has agreed to the ruling, the ruling is now final."