A fan captured the encounter as Mr. Met headed into the tunnel during the 7th inning -- claiming he was just trying to get a high 5 -- but got only 1 digit in return.

The Mets called it an "inappropriate action," and issued an apology saying "we do not condone this type of behavior. We are dealing with this matter internally."

The team later confirmed that the person wearing the costume Wednesday night has been canned from the position.

One thing to note: Mr. Met -- who only has 4 digits on each hand -- doesn't actually have a middle finger. But yeah ... we get the point.