Keegan Bradley said it felt like "a dream" after beating fellow American Jason Dufner in a play-off at the US PGA to win his first major.
The 25-year-old triumphed by one shot in a three-hole shoot-out after the pair tied at eight under in Atlanta.
Bradley had triple-bogeyed the 15th to hand Dufner a five-shot lead but kept his nerve as his rival faltered.
"It feels unbelievable, I can't believe this trophy is sitting next to me," said Bradley.
"It seems like a dream and I'm afraid I'm going to wake up here in the next five minutes and it's not going to be real."
In a tournament filled with unfamiliar names at the top of the leaderboard, Bradley delivered an unforgettable finish.
The rookie was five shots behind with only three holes to play after his chip raced across the 15th green and into the water, leading to a triple bogey, but he refused to let it throw him off his game.
"I just kept telling myself, 'Don't let that hole define this whole tournament,"' said Bradley.
He proceeded to make back-to-back birdies, including a 35-footer that rattled into the cup on the 17th, while leader Dufner saw his game fall apart.
Unflappable all afternoon, he hit his tee shot into the water on the 15th for the first of three straight bogeys that led to the three-hole play-off.
Bradley birdied the 16th hole in the play-off to take the outright lead and went on to win by one shot.
He is only the third player to win a major on his first try and his victory breaks the United States' longest drought at major championships, which had reached six since Phil Mickelson's win at the 2010 Masters.
"I don't want to be one of the guys that kind of disappears," said Bradley.
"I would love to be up in a category with the best players and be mentioned with Phil Mickelson, one of my idols. I hope I don't disappear. I don't plan to."
Bradley, nephew of LPGA great Pat Bradley, went into the tournament ranked 108th in the world but his win lifts him to 29th in the rankings.