PGA Tour golfer Danny Lee says his 6-putt temper tantrum at the U.S. Open was completely "unprofessional and foolish" ... and now, the guy is apologizing for it all.
If you missed it ... Lee went BERSERK on the 18th hole at Winged Foot on Saturday -- throwing a Happy Gilmore-esque fit on the green.
After Lee missed a short par putt, he missed the ensuing bogey putt as well ... and then became visibly pissed off.
So much so, Lee halfheartedly tried to make his next THREE putts from close range .