The waitress at Upper Deck Ale & Sports Grill in the Miami area struck first yesterday -- posting a copy of Warren's credit card receipt where he specifically wrote a big, fat ZERO in the tip spot.
Warren Sapp admits he stiffed a waitress at a Florida sports bar yesterday ... saying the reason he left her NO TIP was because she was terrible at her job.
He also wrote a note, "Boys don't tip."
For the record, Warren had ordered a soup, chicken wings, a sandwich and a bucket of Heineken, totaling $69.39.
Now, Warren is DEFENDING his non-tipping action -- saying the waitress kept calling Warren and his crew "boys" -- a term that pissed him off -- so "the tip fit."
Sapp also says he went to the bar to watch the U.S. in World Cup -- but LEFT before it started because "it was horrible and the service was the worst."
In a related note, Sapp filed for bankruptcy in 2012 and his Nike shoe collection was auctioned off in 2013 ... AND he never paid Rick Ross after losing a $100k bet he made on the Super Bowl.
So, is he REALLY taking a moral stand ... or is he just too broke to tip?