PGA's Danny Lee Apologizes For 6-Putt Tantrum At U.S. Open, 'Foolish'

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 ... before finally slamming home the fourth one.

In total, it took Lee NINE shots to complete the hole ... and when he walked off the green -- he threw his putter angrily against his bag. Lee then withdrew from the tournament, citing a wrist injury.

On Tuesday, the 30-year-old said he was sorry for the whole scene ... explaining, "My frustration took over me and combined with injury I had to fight with it for all week."

"Still just an excuse," Lee said. "I shouldn't left like that... and also like to apologize to USGA they did a tremendous job last week at wing foot. On the course and off the course."

Lee continued, "Now I gonna take some time off and think about what I did and starting next time I'll show up as a better person and have better sportsmanship."