Dave Portnoy isn't joking around about an antisemitic sign that appeared in one of his bars last night ... posting a video screaming about how upset he is -- and promising everyone involved is going to be educated.

The Barstool Sports founder took to social media Sunday to address a viral video going around of a waitress at Barstool Sansom Street in Philadelphia holding a sign which reads, in all caps, "F*** THE JEWS."

Dave explains the sign isn't something a staff member came up with ... but one provided when you purchase bottle service -- with a message the buyer crafts.

However, he acknowledges this one totally crossed the line ... and, he's pissed nobody who works for him stopped the message from going up.

Portnoy says he spent all of Sunday morning tracking down everyone involved ... admitting he spoke to one of the waitresses, who he says has to be one of the dumbest people he's ever spoken to. He adds that he believes they've already been fired.

Dave seems to have cooled off some in the last couple hours ... 'cause he said he spoke to a couple of the people involved in the genesis of the sign, and he plans to send them to Auschwitz to hopefully learn from this experience.

Portnoy's working with Patriots owner Robert Kraft, cofounder of the Foundation To Combat Antisemitism, he says ... and, together, they're financing the trip. Unclear if either of the waitresses are getting this level of grace, but, it certainly doesn't sound like it.