The pro golfer was teeing off Sunday in Adelaide -- where LIV Golf is hosting a tournament with a sellout crowd in attendance -- and on the 12th stop of the course, he hit a hole in one ... which is pretty impressive considering it's a par-3.

The spectators thought so too ... so much so, in fact, that they started chucking their beers, cans and cups right at the dude from up in the stands. A literal beer shower, if you will.

CK didn't mind it one bit ... he was absolutely stoked and reveling in the brew -- rightly so. BTW, the whole beer-throwing thing here is a tradition at this hole in particular ... it's apparently known as the "Watering Hole," so the athletes know what they're in for.

Koepka commented on the drizzle later on, and it didn't bother him one bit. He's quoted as saying ... "I smelled like beer the whole entire rest of the day. Yeah, it was wild. Crazy experience. I mean, obviously to do it on a hole like that was super special."

The American golfer is far from the only star to be out there at the Grange Golf Club this weekend ... he's joined by other stars in the sport like Ian Poulter, Paul Casey and Phil Mickelson -- who abandoned the PGA and linked up with the Saudis for this new league.