Barstool Sports president Dave Portnoy ain't fazed by going up against a 4-time major winner on Tuesday ... telling TMZ Sports he's gonna whoop golf superstar Brooks Koepka's ass on the links -- for charity!!!
El Pres is facing off against Brooks (who will be playing left-handed) in a $250,000 match on September 7 ... with the winner getting the huge chunk of change sent to the cause of their choice.
We spoke with Dave at the Slice app pop-up at Brooklyn Pizza Masters ahead of the big event .